how does one interpret the following statement.

1.All animals have hearts.

does it mean that each animal has more than one heart or each animal has only one heart or it could be both.?

thanks

Great question. It can be both. Avoiding this issue will plague you for the rest of your writing career. You can usually use the word "each" to help eliminate ambiguity.

Good luck, and welcome to the single/plural-mismatch club!