I have seen a team I was working in continue to hiring all the time to find great project players proven with customer source code we worked on. For fixed release date it works as for me, my team grown up to 30 people, 5 of them done features other helped them with boring but also important tasks.
Recommended reading:
https://en.wikipedia.org/wiki/The_Mythical_Man-Month
Though it was written in the 1970s, the way that people write software and the invisible laws that govern that process still hold today.
This book describes those invisible laws and is worth reading if you are managing software development.