The answer might not be as straightforward as you might think, as it depends on what type of fish you intend to keep.

The most fish you can fit into any given size of tank are FRESHWATER TROPICALS, followed downward in number by FRESHWATER COLDWATER fish (Goldfish) and then TROPICAL MARINES.

A rough guide is to allow 12 sq inches per 1 inch body length of fish (don't include tails in any measurement) for TROPICALS.
So, a tank with a 24” x 12” water surface area (288 sq inches) could hold 288 divided by 12 'inches' of fish, ie, 24.

At an allowance of 24 sq inches per inch of fish for GOLDFISH, the number for the same tank would be 12 inches of fish – only three or four modest sized fishes.

At 48 sq inches per inch of fish for MARINE fish, the number for the same tank would be only 6 inches of fish – only three or four small fishes.

This is why most guides advise on a three foot long tank for Goldfish and/or marines.

These figures need to be used in conjunction with common sense.
In the first example although a 24" tank theoretically supports 24 'inches' of fish you wouldn't be doing the fish (or the system, much good if you decided to go for two 12" fish in the tank!

Then there is the question of growth.
What size are the fishes likely to reach when adult?

Remember, all fish on sale at the aquatic shop are juveniles and will certainly double in size in most cases with the correct care. You can find out the expected sizes of aquarium fishes in the FBAS Booklet No 6 - National Fish Sizes. (see details how to get a copy HERE).

Finally, don't be in too much of a hurry to stock the tank- certainly not to its full capacity right from the start. The aquarium conditions, particularly those of the filtration system, need to acclimatise themselves to each increase in bio-mass so do it slowly.