Use all 6 (I think - not counting DPI buttons) on my G502 mouse.
I'm prolly weird, but I like to stick to the following general controls depending on the game:
Thumb button - Misc special actions depending on the game, like spotting/marking in CoD Warzone
Forward thumb button - Melee actions, flashlight, grenades
Backward thumb button - Prone/crouch
Middle mouse button - Reloading
I use 3 of the 9 buttons on my G604. I have 2 of the side buttons mapped as middle mouse click so technically 4.
Back in the day when I was playing an FPS game and scoping in with a sniper rifle was middle click, I would sometimes scroll a little and it would activate weapon switch.
I just bought the new COD Black Ops and I am going to start using a few more buttons on the mouse.
That's it. Not even wheel button is used. I like my controls simple and transparent, an no chance of error. (wheel down can produce occasional errors for example...) My left hand on the keys does more heavy lifting with buttons.
I'm between 2 and 3. In most games I only use left and right click. Left of course attack, right being aim / zoom / similar function. In a few games where there is a weapon wheel I use the middle button for that. I don't like using it for anything else, as I cannot reliably and confidently press it. The rest of the buttons are even worse. It feels awkward to try to press any side buttons, I need to focus on it and I easily mix them up*. Even when using the back / forward buttons during browsing I'd accidentally switch to the other one without noticing it.
*for the same reason I cannot get used to game controllers of consoles. I always mix up the buttons levers and switches in the heat of the action.
Press down on scroll wheel
I have a back/forward button that I map for some games. In CoD MW I use the forward button to mount my weapon on a surface. I know I used these in Watch Dogs 1/2 for something to.
I never graduated from using the arrow keys for movement to WASD, so I use a mouse with a lot of buttons to hide the fact I can't reach most of the keys on my KB.
Middle Mouse Left/Right
Four Side Buttons
For a FPS:
Left Click: Shoot
Right Click: Aim
Middle Click: Melee
Middle Mouse Left: Non-Lethal Grenade/Skill 1
Middle Mouse Right: Lethal Grenade/Skill 2
Side Buttons (Top Row): Action & Reload
Side Buttons (Bot Row): Jump & Crouch
I use RShift for Prone (if necessary) and \ for Sprint (If Double Forward to Sprint isn't a thing). That's generally enough buttons to get things done; I use the number pad for anything else for the most part.
I use a controller for everything unless it's a table top RPG or a MMO with a ton of spells.
I like mice with the center button as a click and left and right scroll on the mouse button and about 3 buttons on the thumb.
For me it's usually setup like this:
3 (Wheel Button): Action/Use/Open
Lower Thumb Button: Aim
Upper Thumb Button: Sprint
Top Thumb Button: Melee
Both buttons to the left of 1: Grenades
Wheel: Swap weapons
My G502 actually has 8 buttons, but I map both of the little inset buttons to grenades. It's too easy to get those mixed up in the heat of the moment, so I just have them both do the same thing.
Interesting that most people aren't using many. I'm guessing that most people are mapping most functions to the KB instead and your left hand is doing most of the work?
I started as an arcade and console player to to me my left hand is for movement and the right is for buttons/functions/etc. I'd take a mouse with even more buttons if they were spaced out properly. Most of the ones with loads of buttons pile them into one spot. I'd love a mouse with 4 buttons across for my ring finger and pinkie, too.
Logitech G604, use all the primary and some GShift buttons. Doom Eternal force me to actually start to use them for me to survive, age thing maybe. GShift usage I've find cumbersome, wish there was a dedicated button on the right side of the mouse for this vice choosing one of the other buttons for it. Over 14 buttons including GShift modified buttons.