It should be noted that the amount of five future PTLs was used as a base for promotion but there is nothing which says that you have to stop there.
If for example, you stop at the five, what happens if you unfortunately lose two?
If one has the time and capacity to train ten future PTLs, he/she is free to do so. In other words, there is no restriction on any Team leader in this regards. Feel free therefore to use your capabilities to create as many future PTLs as possible.