Jon of All Trades has the right general idea but I think the numbers are a bit off.
5% sounds incredibly high, that would imply an average life expectancy of 20 years old. Life expectancy at birth was 35+ so death rates would have been closer to 2.5% per year. Add to that, not everyone got a grave to themselves. Paupers, prostitutes and babies of fairly poor people who died at a young age would have been piled in mass graves
This is what the modern version looks like, in medieval times they generally wouldn't have bothered with the separate coffins for the very poor:
These types of graves take a lot less work per person.
Also it wouldn't take two professionals a whole day to dig a single grave, one professional could probably dig more than one in a single day but lets assume at least one grave.
mass graves are probably 5 or more times more efficient.
So call it 12K dead per year, lets assume half of them are too poor to afford a private or family grave.
So 6k normal graves and mass graves for another 6k.
At one grave per day per gravedigger the private graves mean 6000 man-days and the mass graves mean 1200 man-days.
Lets assume 300 man-days per gravedigger per year for the sake of round numbers giving us about 24 grave diggers.
But we're assuming perfect efficiency and good organisation so lets double that, call it a round 50 full-time gravediggers