While Taemyr's answer is correct, it covers the version of time travel where there is only one universe, and any time travellers must travel into their own past. While this is a common model to use for time travelling stories, I'm more of a fan of the system used in the movie 'Primer', wherein every time you travel to the past, the universe splits off into a new future.
In this case, let's say you're in the field and no new time machines show up. You are the 'prime' version of yourself; the universe has not yet split up, no other universe has sent a time traveller to you. Either that, or the time machine doesn't work.
Anyway, the prime you, let's call it Y1, steps into the time machine and goes back five minutes. When Y1 arrives, they get a chance to meet a new version of themself, let's call them Y2. Y2 was the exact same person as Y1 up until Y1 arrived in the past, but now they are two separate people.
There are two options at this point: either Y2 decides to go back in time like Y1 did, or he doesn't. Y1 could convince him not to; somewhat paradoxically, the fact that Y1 convinces Y2 not to go back in time may lead to the very conditions that led Y1 to go back in time, leading to an infinite loop of Yx going back, then Yx+1 not going back. But let's assume these time travelers are willing to test this out, and they both go back in time. Assuming the time machines end up in the same tangent universe, after the journey there will now be three time machines, and three copies of yourself, Y1, Y2, and Y3. This can continue for as long as Y1 is willing to keep going; it's possible the effects of time travel will cause him to stop eventually.
The interesting part about this model of time travel is that you are not guaranteed to enter the loop at the beginning. You could show up in this field and be met with a thousand copies of yourself, or none at all. They could have been travelling for days, or years, or maybe it was the first jump. That's part of the reason I prefer this model, because virtually anything could step out of that time machine.
Anyway, I'd like to point out that in this scenario, the number of time machines increases linearily; that is, you have one, then two, then three, and so on. However, if you intend to mass-produce time machines this way, there is a very good chance you'll get in on it partway through, so your initial gains could potentially make up for this slow growth. Also, no one but the time travellers and the residents of the last universe are going to benefit from this; for every other universe, every iteration of Y has been erased permanently, along with all the time machines. So it's probably a safer bet to gamble on getting just the second machine, as it's 50/50 odds on double or nothing.