35-40 is generally a good number, provided the abilities are good ones. Anything more than that and I start having to really work to find keybinds for them, many of which are just uncomfortable to use and do not really get more comfortable with time. I wouldn't try to really pick any number as some sort of target and I think designing that way, as opposed to making sure the kit feels complete and well-rounded (at least to the extent it's supposed to be), would be a very poor choice. I do generally like having more skills rather than fewer though, as it usually opens up more ways to approach a given situation in the game and a lot of those abilities can be somewhat niche abilities that really enhance the flavor of a class. However, it was more important to me in WoW than in this game, which has a lot to do with what abilities exist and how they work.