You should have around five Siamese algae eaters. Siamese algae eaters are schooling fish and love living in small groups. Keeping five or six Siamese algae eaters will allow them to express their natural behaviors comfortably.
Make sure they have plenty of space to swim around. The first fish needs at least 25-gallons of water, and each additional fish requires 10-gallons of water. You’ll need a 75-gallon fish tank or bigger for a school of Siamese algae eaters.
Alternatively, you can just keep a single Siamese algae eater if you’re limited on space. Despite being schooling fish, they actually do well on their own. In fact, keeping just one Siamese algae eater is better than cramming five of them in a tiny tank.