Old thread but you can actually have more than 9 actions binded at a time, they just wont be on the same hotbar.
In the skills menu you can go to different pages and thisll let you swap to different hotbars where you can put more actions, you can hotswap these hotbars with the PageUp and PageDown keys. You can also edit how many hotbars you have (useful so you arent swapping through empty bars) in the settings menu. You can have up to 9 of them