Why there are only 2 self-manually employed co-workers out of 4 at my Hospital?

As @jpkarlsen said. The game picks random co worker and sometimes it will spawn new co-worker. I have 8 NPC which work in hospital career. Most the time I got my created, but the game will still create new NPC as my co-workers just because it can. (You can delete this NPC but the game will create a new after some time. I used to delete this NPC but now I just give them a make-over. Normally it takes a while before the game create a new co-worker. I have still the same after 48 sims days)

Your hospital gets 3 co-worker; 1 nurse, 1 doctor and 1 receptionist. They all do nothing in a vanilla game. Some days you don’t have a doctor or nurse because of staff *shortage*. I love the doctor career but in vanilla I hate my co-worker because they are useless and the nurse love to block one patient.  I highly recommended to use simvasion *better hospital staff* and scumbombo *Always keep a Doctor at the Hospital* if you play the doctor career. I don’t know if they compatible with the realistic childbirth mod. I don’t use this mod, but I know it’s rather a popular mod.

You best bet is to ask the creator of the mod *stop generating New Townie* what the issue is. Maybe the mod is broken for GTW-careers.