Err, sorry I'm so late to reply. I didn't see this until tonight. Anyway, the following GBA games were available from day one:
Super Mario Advance
Napoleon
F-Zero
Kuru Kuru Kururin
Dodgeball Fighters
Advance GTA
Rockman EXE
EZ-TALK 1-6
Tweety's Hearty Party
Zen Nihon GT Something or other
Winning Post
Konami's Wai Wai Racing
Dracula Circle of the Moon
Monster Guardians
Yu-Gi-OMGWTF
JGTO Japan Golf Tour
J-League Pocket
Power Pro-kun Pocket 3
Play Novel Silent Hill
Fire Pro Wrestling A
Chu-Chu Rocket!
I'm an Air Traffic Something or other
Mr. Driller 2
Momotarou Matsuri
Pinobee's Adventure
25 in total. 30 if you count the EZ-TALK series individually. Pretty crazy, eh?