In 3001: The Final Odyssey author Arthur C. Clarke postulated that the human brain required approximately one petabyte (250 bytes). Compared to today's technology — say, a common 2 TB hard drive — you'd need 500 hard drives to store one human brain pattern.
How much of that is repeatable data? Neurologists are quick to point out that the human brain is not (not, not, not, not, not) anything at all like a hard drive... or a CPU... or a GPU... or any other aspect of digital computing. And that's despite the fact that we don't really now how the brain stores information in the first place.
That means there could be some incredibly high inefficiencies when it comes to converting brain-patterns to digital patterns, not unlike the pain involved with using an analog-to-digital converter to store sound or video. We're going to ignore this, but it might make a cool story element.
So, for the sake of argument, let's assume some completely arbitrary, pull-it-out-of-thin-air conditions.
15% of the brain is automatic processing. This includes motor control, heart beat, thermal balance, and the thousands upon thousands of other frankly magic things the brain does without any conscious thought whatsoever.
25% of the brain is subconscious processing. This includes processing visual, auditory, tactile, and taste information. It's also our ability to walk (including "muscle memory") and otherwise move muscles without conscious regard to what, exactly, we're doing. (Anybody who's driven somewhere and realized they have no significant memory of exactly how they drove the car, made the direction decisions, etc., will know exactly what I'm talking about.) This would also include unconscious processing (aka, dreaming and whatever is actually happening between our ears when we sleep).
25% of the brain is conscious processing. This is the part of the brain that lets us pat our heads and rub our tummies all at the same time. It's what allows for acrobatics and thinking. It's what allows us to learn. This part of the brain could be where the divine gift of reason resides. Maybe.
That leaves us 35% that's pattern storage. This is important. The brain doesn't store bits. It stores patterns. What that actually means is well beyond my pay grade, but it has everything to do with traditional audio/video/taste storage as well as recognizing symbols and actively remembering why you secretly liked Baywatch.
Starting with this entirely untrustworthy baseline (don't bother complaining, I'm just going to point you to this part of the answer), what we might, maybe be able to assert is...
- The automatic processing component is 100% repeatable between humans. Oh, there's those odd little tweaks like heart arrhythmia, the tendency toward arthritis, or full blown we-love-Sherlock sociopathy that could exist — but if you think about it, those are just dials that are turned somewhat randomly in our lives.1
It's worth noting that if you have this ability at all, you also have the ability to cure or cause every known mental condition. That's interesting, don't you think?
- The subconscious processing part is, probably (and IMO), also completely repeatable. Which means you also have control over how people perceive taste, light, sound, etc.
Where things get dicey are those last two percentages. Some of #3 might be repeatable, but #4 is 100% unique.
Remember that the brain appears to store patterns. Those patterns will, of necessity, be based on the unconscious behaviors of the brain. In other words, the information stored in Sherlock's head has as much to do with his sociopathy as it would his training and learning abilities.
Which means you might not be able to repeat #2 and #3 at all.
In fact, there's a philosophical argument to be made that if your memories are based on conscious and subconscious perceptions, then it would be based on the automatic perceptions, too. Like the memory of running flat out for 60 seconds leaving you huffing and puffing — which has a lot to do with #1.
I've just convinced myself that basically nothing of the human brain could be "repeated" or duplicated between people without damaging the core-memory that would define "I think, therefore I am."
So, you need all 500 hard drives per-person. 7.5 billion people on the planet — 3.75 trillion hard drives. And that assumes no growth. Growth is your enemy.
BTW, I'm assuming you're only keeping "snapshots" and not "incremental backups." You've got a problem if you're trying to store incremental backups.
Now that we know a reference point, what can we do about it?
You need much better storage tech than we have today. Stanford suggests you need ~35fJ to store one bit on an SSD today. That's 2.6e11 Joules or 0.26 terawatts of power. If you think about it, in 2014 the world consumed about 109,613 TWH of power. You need your terawatts running 24/7/365, so you need (am I doing the math right, folks?) about 2,300 TWH to run your server or 2% of the planet's total power consumption.
Honestly, that should make you want to pee your pants. That was ONLY the hard drives.
Conclusion: After muttering some arcane sounds and flipping through some ancient tomes you successfully invoke Clarkean Magic and do exactly what you want, the way you want it, without having to worry at all about year 2020 limitations.
Because, baby, if we use the Ginna nuclear power plant as a reference, it generated about 4.7 MWH in 2018. That means you'd need almost 500,000 nuclear power plants just to run the hard drives.
I couldn't possibly have done that math correctly. 500,000 nuclear plants? The only problem is, I have sources for the world power consumption in 2014 and the Ginna plant output in 2018. So unless I missed a factor of 1,000 someplace... somebody tell me I missed a factor of 1,000 someplace....
1 I got this idea from a close friend many years ago who taught a class about autism to some teenagers. Obviously she's simplifying for her audience, but the cool thing she said was that you could consider everyone to be autistic. Autism, she explains, was something like a bunch of dials that were set too high. Too much sensitivity to light, sound, etc. All of us are, to one degree or another, "autistic" and what we define as "autism" in society is actually the worst-case circumstances of the very natural human condition. I found that explanation very humanizing and compassionate (whether it was right or wrong). Anyway, that's where the idea came from.