I have a script on the server that replicates all the players’ head and waist rotation. The client script sends the server the neck and waist rotation data every time their camera’s CFrame changes. If my game had 30 players and there was only one server script that would handle the replication, would this cause issues?
im guessing server lag is a potential issue
For the most part, yes. Having one remote event getting 30 different events very quickly would definitely lag your game. I wouldn’t bother replicating that, however what I would do is I would have a vector value that shows the mouse position in each player, and locally I could update each players rotation data to prevent too much lag. You could also just only make it do it for your own character, instead of everyone else.
Recieve data from the client, verify it, then use :FireAllClients() to send the data to everyone.
I have multiple remote events set up for my 30 players 
but what I was actually asking for was a single script not remote events.
Edit:
Or is it the same thing?
This could actually work, but I worry that this method would cause a significant delay in the replication. Its like going from destination A to B then back to A
The best way would be to have 1 remote event that any player can fire (use plrsent to figure put who fired the event), then frame/ set the values on the server. That way, the client prioritizes it’s own head, but doesnt lag due to others. Do you really need other player’s heads to update instantly? No, the player will only notice delay with themselves, not others.
It probably won’t lag, but you could definitely make it better by only sending camera CFrame and calculate the hip/neck on clients.
Don’t know if this suggestion would be good enough but is worth a try.
You could have 5 animations, looking left,right, centre, up and down. If the player turns their camera enough, it can fire the animation to the relevant place they look at. You wouldn’t have to send events as much as if you were to track the camera’s position and move it relatively.
That wouldn’t mingle well if you have to create other animations that need those limbs. The point of editing the look direction of waists and necks without animations is so that animations can play while the avatar is looking in a certain direction. Such animations would overwrite the look animation and have to.
If the look animation is prioritised over another animation, it’d look odd. If your action animations are prioritised over your look animation, it won’t play.