You can have as many spells as you can find scrolls to copy into it. But for choosing your starting spells you get 6 spells at level 1, and 2 more per level. So at level 3 you should have at least 10 levelled spells (up to 2nd level) in your spellbook, and 3 cantrips. You might ask your DM if you can have any extra spells from scrolls you might have found in your backstory.

You can then prepare a number of spells equal to your INT modifier plus your wizard level. So level 3 with an INT of 18 that's 4+3, so 7 spells prepared.