I would say 3-5x exprate is good, if your Team gain some cash in this project (ads on the website for and example) you can always open a new server?
since lots of ppls will change server well that is how the players on the regular tibia works.
If you dont want to spend money on two dedicated servers you should have it like 3x.
Edit: or just have 3x un till 100 and then take it to 2x and 200+ take it to 1x.
since then ppls will be more careful about the chars at high lvls.