Pictures:
http://imgur.com/308wn5S,YrUDa8B,vVQwA95#2I maxed out at 5 OpenGL/DirectX clients on Smart.
I maxed out at 2 OpenGL/DirectX clients on the official client (some advertising thing blocks you from opening more even if you're a member).
I maxed out at 4 OpenGL/DirectX clients on the browser.
I maxed out at 5 OpenGL clients on Powerbot.
Maxed out at 9 Safemode clients on Smart/Browser.
Maxed out at 12 Safemode clients on Official client.
None of which were logged in. I tried them with and without the plugins. There is a client or hardware limitation. After 4-5 clients, it forces you to safe-mode.
You cannot overcome this limitation by running 5 in OpenGL and 5 in Direct-X. I tried that as well. You can however mix-match them (3GL, 2DX, etc..).
After hitting 5 clients in OpenGL, the Direct-X button is actually no longer available/there on the sixth client and it goes into safe-mode with the only other option being OpenGL mode which it won't let you enter.
It's safe to say that safe-mode/cpu-mode allows more clients whereas the graphics card will only allow 5.
P.S.
I'm not actually sure how you guys are able to run 30 safemode clients.. wtf? I can't click anything after 12-15..