You summon summon any monster but only one at a time according the following table where the number is the level.

You choose a monster type and some key qualities and then the GM summons a monster that fits your qualities list and if one doesn't exist then summons a random one that  fits the type. It is best you don't know the monster's full stats and name. 

You can't control a creature until you pass a Charisma save or otherwise convince the monster to follow you by convincing it or know its true name. 

1. Challenge rating 0 or 1/8
2. Challenge rating 0, 1/8, 1/4 or 1/2
3. Challenge rating as per 2 or CR1.
4. Challenge rating per 3. or CR 2.
5. Per 4. or CR 3. 
6. Per 5 or  CR  4.
7. Per 6 or CR 5.
8. Per 7 or CR 6.
9. Per 8 or CR 7.
10. Per 9 or CR 8. 
11. ....

20. Any CR