As many as your microSD card of choice can hold. A 32GB card (the smallest size most stores carry) can hold well over a thousand GBA games and sells for around $10 USD. Whichever flash cart you choose, they all have a microSD slot (though as Nikokaro mentioned, lower-end models like the Supercard can only use cards of less than 2GB; you can probably find a card of that size in an older-model mobile phone)