A stick could technically support any number of channels if it works and is paired with enough other sticks. A single stick is a single channel. 2 sticks for dual channel. 3 sticks could be triple but the memory controller affects max channels. Most common mainstream can only support 2 max so would be dual plus 1 single. 4 sticks quad, etc etc.
Sticks are sold in kits to guarantee they work together. Mixing sticks usually works but there is always the chance it doesn't.