Yes, otocinclus need to be kept together. Otocinclus are very social schooling fish, and they love living in groups.
In fact, wild otocinclus typically live in groups consisting of thousands of fish. Additionally, they will become unhappy and stressed when isolated from their peers. Therefore, it’s necessary to keep several otocinclus together.
To keep your otocinclus happy, it’s important to keep at least five to six of them together. Also, since otocinclus are small, you don’t have to worry even if you are short on space. A small 10-gallon tank is sufficient to house up to six otocinclus.
That said, if you have the space and resources, it’s highly recommended to keep 10-20 otocinclus together.