This comes from two sources. First is the sorcerer class. At level 3, you can cast zero-level spells 5 times a day and first-level spells 3 times a day. Next you look at the ability score, which gives you bonus spell castings per level. At 21, you get +2 1st level castings.
So at level 3 with 21 CHA, you can cast zero level spells 5 times and first level 5 times every day.
Now, the 21 CHA gives you a +1 to spell levels 2nd-5th. However, you cannot cast these spells until your character class grants you access to those levels. So you can cast zero 2nd level spells until you are a 4th level sorcerer (at which point, assuming your CHA stays the same, you'll be able to cast 3+1 = 4 second level spells a day).
Just so you don't get confused about 0-level spells:
Cantrips: Sorcerers learn a number of cantrips, or 0-level spells, as noted on Table: Sorcerer Spells Known under “Spells Known.” These spells are cast like any other spell, but they do not consume any slots and may be used again.
So you can pick 5 and cast them as much as you want.
Crossblooded doesn't affect the number of spells you can cast per day.
It affects the number of spells you actually know.
As a 3rd level Crossblooded sorc you get 4 cantrips and 2 first level spells known (+bloodline spell)
You can cast 5 first level spells a day (+cha mod)
At 4th level you get an extra cantrip, and no 1st or 2nd level spells. However the 2nd level spell SLOTS are still open to you (3+cha mod). Good for possible metamagic or just spamming your level 1 spells more often.
At level 3 you would have 3 level 1 spells (2 chosen, 1 from bloodlines) that you could cast 7 times a day (with 21 cha).