First of you should try to get your main abillities keybound in a easy and suited spot, then find a okay spot for defensives, depending on what class you play you might wanna put mobillity abillities around WASD, like Operative roll etc.
I never go further than 5, because its hard to reach ctrl + 6 etc, but I dont even use ctrl
For example my Sentinel, I use "R" combos for my big hitter, like Sweep, Blade Rush or Merciless Slash, and "F" combos for my "oh ****" cooldowns like Rebuke and Saber Ward.
If you play a stealther, I would say dont have sap on i.e Shift + E and have something that would take you out of stealth on E