When setting up an open mic night, be sure to include enough seats for all of the guest musicians who will be on stage at any given time. Even musicians who play standing up, like horn players, may want to sit down while they are not playing. You also need a PA (Public Address) system, which includes speakers, a sound "EQ" mixer, monitors and cables to connect those components to each other and to the electronic inputs. In addition, be sure to have power strips, extension cords, music stands, a sign-up list for guest performers and stage lights if needed.
If you are providing the electronic instruments, you should consider including a keyboard, an electric guitar and an electric bass. Also provide amplifiers and cables to connect the instruments to the PA system and amplifiers.
If you are providing drums, be sure to include a complete set with a bass drum, toms, symbols, a snare drum, the drum stool, sticks and mallets. To avoid slippage of the set while it is being played, you should put the drum set on top of a rug. If the open mic night is taking place in a large room or will be played over a loud crowd, you should consider miking the drums. This requires several mics as well as boom mic stands.
Most acoustic performers will bring their own instruments. However, you may want to have a piano and upright bass available because those instruments are rather heavy for guest performers to bring. In addition, have a piano bench, as well as microphones for each instrument if the room size and noise level in the room warrant them. For vocalists, you should set up a vocal microphone as well as bottles of water. Musicians bringing their own instruments should remember to bring appropriate accessories (e.g., mutes for trumpets).