There is no specific number of members required for an orchestra performance. Orchestra is an ensemble of musicians and musical instruments. There are mainly four groups of instruments played in an orchestra. It is okay to say at least 10 - 15 members will be required for an orchestra performance. The number of members required depends on the musical piece the orchestra will perform.
An orchestra with less than 50 members is considered small and is called chamber orchesta. An orchestra with over 50 - 100 members is called symphony orchestra.