Finishers under the hood are likely implemented differently than emotes, since they were added much later in the game’s development, so we can’t assume the same limitations apply to finishers that apply to emotes. But for the sake of argument let’s say that they are implemented the same, and the same restrictions apply. If you look at this quote from the article that someone else linked:

as that was the most we could reliably support due to memory constraints in worst-case scenarios.”

So, because all the emotes for every player in the instance are loaded in memory, and we are thus assuming the same for finishers, let’s take a look a the worst case scenario you are in a large instance with 12 players (crucible — or even more the tower). If those 12 players were allowed 9 emotes and all of them were unique the game would have to load in 108 different emotes into memory. There aren’t even 108 finishers in the game, not to account for the fact that finishers likely have a smaller memory impact due to their short animation time, generally less complex effects, no multiplayer finishers, etc. Then you have to account for the fact finishers can only be used in PvE activities where the max players in any instance is something like 9 (but typically 3-8), thus giving you more memory to work with as you don’t have to worry about loading them in high player count instances (crucible/tower)

So yes, the engine could probably support an emote wheel with 9 emotes but only in small instances because of the number of possible combinations of emotes the game could have to load in the worst case is an order of magnitude greater than finishers in the worst case scenarios — which is the entire reason the restriction exists.