M315-M386 trios (which may include one or more brass instruments)
M357.4 brass trios
M415-486 quartets (which may include one or more brass instruments)
M457.4 brass quartets
M515-M586 Quintets for various instruments
M557.4 brass quintets
M657.4 brass sextets
M757.4 brass septets
M857.4 brass octets
M957.4 brass nonets
For other combinations (sextets, septets, etc.) follow the same pattern as above
Also try the subject heading Brass Ensembles
For brass ensembles with other instruments (piano, violin, etc.), after each basic subject heading (trios, quartets, quintets, etc.) the instruments are organized in the following order: Keyboard (piano, continuo, etc.), winds / brass, percussion, strings (in score order).
So, for example, if you are searching for trios for clarinet, horn and piano use the subject heading trios piano clarinet horn
Music Reference