This is a great question, and one every brand approaches differently in terms of design. Once you buy a dry bag, there should be a tag (sometimes even printed instructions!) on the top inner portion of the roll-top bag.
The magic number is usually three to make sure there’s an airtight and watertight seal. But the amount of times you’ll roll a bag also depends on the amount of gear inside.
Always be careful not to overstuff a dry bag — there should be room to roll it to fold that seal. On the other hand, if you only have a few items stored in a larger bag, make sure you roll out the extra airspace for easier packing on the water. Alternatively, you can roll the bag a few extra times.