IMHO....number of instruments is up to you...I've watched Sheds on youtube with many different variations....2 drum sets....1 drum set with tracks...2 sets, organ, bass, sets, organ, key board, bass, guitar....etc. I say find out how many people want to come, what instruments they play and get ideas from them. Oh...the size of the space might matter also. What are u trying to accomplish with the session? I noticed you said "conduct"...are u trying to teach something or have fun/increase skills. If it is mainly to have fun, network, etc....then you should have many options..as listed above. After the first you, you will be able to change/amend your setup fro future sheds