Most of development of HOI4 has been a team of 4 programmers/scripters/designers + 2 artists on average. It goes up and down though. Current core team has grown with 2 people now and we do steal resources all over when schedules need to be met for short periods. This week we are somewhere around 10-12ppl on and off for example. For a final answer you should probably look at credits of earlier titles (I didnt count music here for example or QA. without QA you cant make a game. I think we have something like 12 QA right now, but they are of course spread over projects so its hard to specify for hoi4 only)