There is no rules on max reserved posts by thread starters (I could be wrong). I remembered some members said it is restricted to 3 reserved posts only. However, in the past, I made more than 3 reserved posts in my topics but I did not get any issue from moderators.
In fact, from my observation, for signature campaigns or bounty threads, mostly thread starters did not actually come back to edit reserved posts with additional content. Anyway, if rules allow it, it does not harm when you reserve some because you might need them in future.