Don't Sell the Milk Before You Buy the Cow

Don't count your chickens before they hatch is an expression used to describe a large number of eggs which are about to hatch. 20 eggs doesn't mean 20 chicks 20 eggs could be 17 chicks you don't know and it's very unusual for them all to hatch if there's a big number. There'll be some which either don't have a chicken side or the chick sadly has already died before it hatches that's very common in other words  you can't come to a conclusion until you have the evidence there in front of you so wait till the evidence is there before you. Don't sell the milk before you buy the cow is another expression with similar meaning.