Shape genetics are so complex, you're bound to get some surprises when one of the parents is a heart.
Five or six is a good number to grow if you have limited space and you're hoping to pick up a recessive trait, you have a good chance of finding it. If you were looking to recover size, you would need to grow ten or more hoping to get a large fruit in your F2 or F3. But if you don't have space and you don't find what you want from 5 plants, you can always try again the next year with the same generation. And there's always luck!