That depends on project duration. Most projects did not go on more than a year or two. If that's the case your ramp up would happen once every two years even if everybody stayed forever. If you're reasonably efficient about it, you can hire someone with maybe a few hours of engineer time. If the average engineer stays even a year it's not that much.

You don't bother thinking about that. Over time you have a large enough sample size for attrition rate and you hire based off that. it doesn't matter if a few leave, a few will always leave.