For optimal mining be sure to check this guide.

At least you will need to provide up to three different witnesses/miners in your config.ini file.

The reason that we have to set up multiple witnesses is because of something very interesting about the way coins are mined on steem. I will get into exactly why in a bit, but roughly the reason is as follows. Once your miner hits the "target", it lines up to get in the queue to mine a block. The queue currently averages about 100 people in line and takes about ~1.5 hours to get through. When your first witness gets in the queue, it stops mining. It doesn't look for any more hashes. This wastes time, so during these ~1.5 hours you want another miner waiting in the wings. That's why you set up multiple witnesses. Just thought I would mention that because no other guides seem to have mentioned that. Another thing to note is that the witnesses are selected in alphabetical order.