Good topic!

Two hot bars are enough. Anything more than 1 through =, Shift + 1 through Shift + = is excessive. So, to answer the OP's question, twenty five abilities or more are too many skills.

Yes, by using Ctrl and Alt, you can double the amount of skills available but can an average player actually keep track of that many different skills? Humans aren't that good at multi-tasking or remembering more than seven digits in a row.

This is good instance where less is more. Too many skills results in many skills being forgotten in the heat of battle. It's much better to have a smaller set of skills which will be constantly used in combat which is why two hot bars full of abilities are a good amount of skills. Twenty four are enough slots to have your combos, utilities, and situational abilities without overwhelming the player with choice.