How many beds do I have?

If you don't have a bookkeeper, or don't have one who's been trained up enough to have perfect accuracy, there are some other ways to find the information you're looking for.

First, when you do place a bed, make sure it's set up as a bedroom: query to select the bed, resize to the size of the room you want, then Enter to set it. If you then leave it alone, dwarves without bedrooms will eventually claim the room, and they'll have a bed.

Your new migration wave will eventually claim any unclaimed bedrooms, and have their own beds. You can check to see who has a bedroom already claimed by going to a bedroom you've designated, but is still unclaimed, then query the bed for the bedroom, and assign the bed. The green dwarves are the ones who haven't yet claimed a bed. You can scroll through this list to see how many dwarves don't yet have bedrooms.

Finally, you can see how many beds you've have created, but haven't yet built by using build, then bed. The screen will show "Placement", if there are beds to be built, or "Needs bed" if you have none left un-built. If you have some to place, arrow to your location you want the bed, Enter, and then you'll see a list of the beds to choose from. They'll be listed as something like "acacia bed, dist 26, 0/8", suggesting there's an Acacia bed 26 steps away, and you have 8 of them un-allocated somewhere.