In a watermelon, there are about 100 - 300 seeds in it.
Seedless watermelons are a special case: They have much fewer seeds than normal watermelons, but they contain at least a few pale, soft seeds.

If you want to plant watermelons, it's the best to plant 3 seeds to get good germination.
After that, select the strongest one and pull out the others.

But: Most store-bought watermelons are from an hybrid plant - there you might not get any fruit if it grows.