I have 12 panes in a tmux session. I created them by clicking ctrl + b
I know how to access panes 0-9. I clicked ctrl + [0-9]
How do I access panes 10 or above? It automatically switches panes after I click the first number.
Do you mean windows or panes? If windows you can access higher numbers by doing C-b ' then typing the number, or by using tree mode with C-b w. Or you can turn the mouse on (set -g mouse on) then click on the window name on the status line. Or by using C-b n and C-b p to move forward and back between windows.
If panes, then you can navigate with C-b then the arrow keys or turn the mouse on then click on the pane.