There's no hard and fast rule about stocking levels. It comes down to the fish being kept, water changes, feeding schedules and tank size. All of these factors have an impact on how many fish can live in a tank.
If you have a well filtered tank that is 2 foot long x 2 foot wide x 6 inches high, it holds about 50 litres and you could keep 30-40 male guppies in it if you do regular water changes. Change the shape of the tank to 2 foot long x 1ft wide x 1 ft high and you get the same volume of water but you would only keep 10-15 male guppies in it, with regular water changes.
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If you have lots of hiding places (plants for guppies) and there are no females to fight over, then you can hold more fish compared to tanks with fewer hiding places.
If you have males and females in the tank, then they need a bit more space due to territorial disputes over prospective partners.
If the fish are peaceful schooling fish, then you can keep more together. If the fish are highly territorial species, then you have fewer fish in the tank. The schooling fish don't mind having lots of tank mates around them but territorial species will not tolerate having others getting in their face all the time.
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If you feed the fish 5 times a day and do a 20% water change once a week, you would reduce the number of fish in the tank. If you feed the fish less often (1-2 times per day) and do a 75% water change once a week, then you can have a few more fish in the tank. If you feed the fish 5 times a day and did a 75% water change every day you could hold lots of fish in the tank.
More food going in means more nutrients and waste products, and you have to do more water changes and gravel cleans to compensate.
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I'm not sure on your tank's dimensions, but if it's 18-24 inches long x 10-12 inches wide/ high, and you have 5 male guppies and are feeding them a couple of times a day, and you do a 50-75% water change and gravel clean the substrate once a week, and the tank is well filtered and has some plants in, then it's probably not overstocked and you could add some other fish. However, if you have a group of 5 male guppies and they coexist happily, do not add any more guppies to the tank because you will upset the pecking order. If you want more things in the tank, add snails, shrimp or some bottom dwelling fish that won't compete for space with the guppies.
Make sure you quarantine any new fish for at least 2 weeks before adding them to the tank.