I wouldn't do back-to-back days of serve practice. That's just me being cautious because I came back from two shoulder surgeries already (I'm 27 and I had those surgeries at ~17 and ~22)
This is just from personal experience and I would suggest the same to you guys, simulate "real" match-like stamina in your serving practices so you don't burn yourself out. That is, after X amount of serves, take a short (~10-30 seconds break). Think about it, you would never hit 20+ serve nonstop in a real match. It's 1 or 2, then play the point, and serve again.
For me, I serve out to a love game nonstop, 1st+2nd serves. So after 8 serves, I walk along the baseline for ~30seconds to give my rotator cuff muscles a rest (like they would in a real match after hitting a serve). Then back for 8 more.
I can go ~200 serves of mixed variety. But if I'm working on my kick serve, I cap it at ~75. Again, I would not do back-to-back days of serving practice.