I recently bought a plot of land and built the manor 🏠 which, once semi-built, comes with two small beds. I have a bard, a driver, a steward, a housecarl, and two kids. Does this mean I don’t need the bedroom or will I be unable to let my spouse move in with me without it?
If you want your spouse to live with you in a Hearthfire manor, then yes, you do:
"In the base game, a spouse will move into any house you own.
For Hearthfire houses, you must have built a bed for your spouse."
Do note that you don't need to own a house at all: marriage is possible without you owning property, in which situation you can live with your spouse (or 'betrothee').