Anyone have an idea to check the markings?
Pour water, a gallon at a time, into your fermenter. Use a sharpie to mark where one gallon is. Then two, three, four, etc.
If you're making 5 gallons of beer, that's 640 ounces. It's not uncommon to lose about 1/2 gallon, give or take, to trub in the fermenter.
That means that 575 ounces or so to bottle. That's about 48 bottles.
If you have less, that's fine if the beer tastes great.
But you can figure out where your loses are, if you want. See how much you're leaving behind in the fermenter, by measuring it. If you're using a clearing vessel ("secondary"), maybe you're leaving 50 ounces behind there. That would explain the lower volume.
But really- if the beer is good, who cares?