Obviously bare in mind that there are species of catfish labelled "bumblebee" cats that range from 3.5" to over 24".
The Dwarf Bumblebee Catfish - Microglanis iheringi is the most popular and stays under 4" long whilst the other fairly common one is the Giant Bumblebee Catfish - Pseudopimelodus bufonius, which gets 9-10".
Look these 2 up along with the others on planet catfish and see which one looks most like yours, i would suspect it is one of the Microglanis species.
If so they are not particularly fussed whether they are in groups or not, although obviously they will be more active if they are in a group.
Ben