Team is growing from 5 to 19 heads. Hopefully the work load per person goes down after they're up and running.

Anyone had this happen to them? Any dynamics to watch out for?

Just one. It will all go to shit pretty soon

I hope you’re well organized for this change: ie split into smaller teams with well defined perimeters. The risk is much higher that people are stepping on each other’s toes. More communication is necessary. More PRs are opened. Don’t get lazy with reviewing other peoples code otherwise a lot of shit code will potentially make it to production.
All in all, you’ll notice that each dev taken individually will produce less, since more time is spent on coordinating everyone’s efforts

Who's gonna train them?

When the head count increases, it is not usually to ease the pressure on the existing ones. It is most usually to add more and bring more chaos

Yes heads grow but progress not always follows..

@retoor likely me. I'll be doing the training.

@lungdart I once got hired together with 5 other people. No one trained us, I was the only surviver