I use Pentadactyl (formerly Vimperator), and hide the tab bar entirely. 'Number of tabs' then becomes pretty meaningless, as all tab-switching is based on a history stack and regular expressions (a la Emacs buffers), so one just lets the buffer list grow. I only notice when there's a noticeable slowdown (which on this Core i7 with 3GiB RAM starts around 180 tabs).