If more than three off-topic close reasons are ever allowed then one idea to keep this in check would be to allow unlimited off-topic close reasons, but require every close reason after three to bump up the number of moderator approvals to get reasons approved.
Thus the first three reasons would have to be proposed by one moderator and approved by a second. The fourth reason would require two other moderators to approve it and require the other three reasons to be approved by a third moderator before it would go live. The fifth reason would require three moderators to approve and all four other reasons to be re-approved, and so on.
This system would make it increasingly difficult to add an arbitrarily long list of off-topic close reasons. It would require every new reason to be considered in light of the existing close reasons, and it would require greater community involvement and support to convince enough moderators to approve new close reasons.
It would also mean that sites which only have 3 moderators would require community moderator support to add a fifth close reason.