I recently bought a couple of gaming mice with 12 thumb buttons to compare... a Logitech G600, and a Redragon Perdition (M901), both off Amazon. The G600 has the ability to program one button to change the functions of all the other buttons, doubling the number of programmable buttons available, but unfortunately the G600 can only store multi-key macros in its Windows software. The Redragon Perdition stores macros in its onboard memory, so once you program it with the (buggy, inconsistent) Windows software you can use the macros on BSD. The macro programmer can accept a maximum of 31 keypresses, or 16 keypresses if you want a delay between them. I've programmed it to speed up mouse browsing with Firefox so I don't need to reach to the keyboard with my left hand for various functions. I think Redragon have released a later mouse they call the Redragon Impact (M908) that I found on their international site
www.redragonzone.com but couldn't find on
www.redragonusa.com but it's also available on Amazon for about the same price, ≈$30.
The first thing to do with it is turn down its fancy RGB LED lights, disable all the DPI settings except one so it has a fixed sensitivity and you can use the DPI switches as extra buttons, and remove the mouse weights from under the cover on the bottom (unless you like a heavy mouse).
There's another mouse I'm going to try called the Rytaki R6 that I think has two buttons beside the first mouse button (redragon perdition only has one), and I have uses for another button so long as it stores macros in internal memory.
Here are some images: