[MikeBmusic]

When starting out, you can get away with less equipment, but the most important thing is finding a venue - a place that will PAY YOU to dun the open mic and has a built-in clientele. If a venue expects you to fill up the room with your open mic attendees, you won't last long.
You need to advertise your open mic wherever possible to get people to come, including inviting all your friends - and telling them to tell people.
Since you mention amps, are you considering a jam (where people play together)? If so, it'll be tougher to get going unless you know people now who are interested in doing this.
Unless it's a jam, advertise it as an 'acoustic open mic', forget the amps, just user mixer and powered speakers (or amp/unpowered speakers, if that's what you have for a PA).
Music stand with a clip-on light.
2 mics and mic stands
Cables (and an extra XLR and 1/4" instrument cable, just in case)
Sign up sheet (on a clipboard or note pad) and pen.
In the many open mics I ran, there was only 1 regular performer whoever needed my soundhole pickup, 95% of the time, I brought it for nothing.

What I did buy (and now use all the time for myself) was a Radial PZ-Pre - it allows plugging 2 guitars in at a time with either/or/both option, but most importantly, has a mute switch. I keep it at the edge of the stage near the mixer, so can quickly mute someone before they unplugg their guitar!