A solid panel wider than the boards you can buy is made by gluing narrower boards edge to edge. The arithmetic looks trivial until you remember that both edges of every board have to be made straight first, and that costs width on every board rather than once on the panel.
Jointing loss is per board
A rough board has two edges that are not straight, not parallel, and not square to the face. Both have to be corrected before a glue joint will close.
That correction removes material. On reasonably straight stock, a quarter inch total across both edges is a fair allowance. On a board with a bow along its length, it can be half an inch or more, because you are cutting a straight line out of a curve.
The important point is that this loss happens on each board, so four boards lose four times as much as one. Working from the nominal widths and discovering the panel is an inch narrow is the standard way this goes wrong.
Narrower boards give a flatter panel
It seems obvious to use the widest boards available — fewer joints, less work. In practice the opposite gives a better panel.
A wide flatsawn board cups noticeably as it moves, and nothing in the glue-up stops it. Ripping wide stock into strips three or four inches wide and regluing lets you alternate the ring direction, so the cupping of each strip opposes its neighbour.
The panel still moves in width — nothing changes that — but it stays flat, which is what actually matters for a tabletop or a door.
The cost is more glue joints and more jointing loss. On a piece that has to stay flat for decades that is a good trade.
Grain and colour matter more than the arithmetic
Before gluing anything, lay the boards out on the bench and shuffle them. Match colour across the joints, and try to run the grain in the same direction so a handplane or a sander does not tear out on one board and not the next.
Mark a big triangle across the whole panel face once you are happy with the arrangement. It is the only reliable way to get the same order and orientation back after four boards have been handled, jointed and carried to the clamps.
Boards that look identical on the rough face can be wildly different once planed. Take a card scraper to a corner of each and look before committing.
Clamps, and how many
Figure a clamp roughly every eight to ten inches along the length, alternating above and below the panel. Clamping from only one side bows the panel toward the clamps.
Pressure should be firm, not maximum. If a joint needs heavy pressure to close, it is not jointed properly and the glue line will be starved where the wood is fighting back. A well-jointed edge closes with hand pressure alone.
Glue squeeze-out along the full length of every joint is the sign the joint is closed. A dry patch is a gap.
Biscuits, dowels and splines
A properly prepared edge joint in solid wood is as strong as the wood itself without any reinforcement. Biscuits and dowels add alignment, not strength.
That alignment is genuinely useful on a long panel where boards want to slide out of plane under clamp pressure, and it saves flattening work afterwards.
What none of them do is stop the panel moving in width. A biscuited joint moves exactly as much as a plain one.