When you're mixing live sound, open only the microphones that you need.
With a sound reinforcement system, the more open mics you have, the less chance there is for you to increase the gain of the system before feedback occurs.
With two open mics, you will have 3 dB less gain before feedback than if you had only one mic open.
With four open mics, you will have 6 dB less gain before feedback, and with eight open mics you will have 9 dB less gain before feedback, compared to only one open mic. (3 dB less gain for every doubling of microphones.)
If your sound system isn't designed to accommodate the number of open mics you plan on using, the system may not be able to provide enough level to the audience.
