In order to have 10 sheep at once you first need to expand an area 3 or 4 times. In the top right corner you can see how many sheep you currently have and a bar below indicating your limit.
The best way to get more sheep is to feed them red flowers, drag the sheep together (a male and a female) then drag the heart above into a cloud and repeat.
Using tombstones which require only 5 stars for a revive helps a lot with this (5 stars a sheep). Good luck!