Me personally, I have been there many times without enough channels. You could run out of one mixer, into another but your best bet is to buy an 8 channel with phantom power. There are many many options out there. I have a 4 channel from Sound Craft. I like it. They make an 8. I also lived by Yamaha for years. Behringer is a great solution and affordable.

Check out the: Behringer Xenyx X2222USB Mixer with USB and Effects
This thing has EVERYTHING you need, including an Interface. $320

But you have $600, this is what I'd like to get, but the 16 channel version. This has everything as well as a recording feature that's super intuitive and very handy. It has an SD Card slot. Slap one in before the show, hit record, and you're able to post mix if you have your levels right, you'd be amazed at the recordings.

TASCAM Model 12 12-channel Analog Recording Console.

Both of these have 2 Aux sends incase you ever wanted to add monitors, you can run 2 different mixes.

We're pretty basic, we need up to 8 channels for instruments and mics for Contra dance bands and caller. No need for reverb and other fancy stuff, that's all beyond my pay grade.
I see a few set-up for ~$100 range am concerned about reliability etc.

From my limited 40ys of experience in PA’s and the recording industry. If you need 8 channels, get 16!

Dennis - I'm no longer very active in live sound, and haven't bought a mixer in a long time, but here are some general thoughts.
You could just buy an additional mixer and plug it into a line input (or AUX input) of your existing one, but that's not the best solution. You might lose a mic channel if you have to plug into a line input, there's more stuff to carry around, more connections to make, and it's better if all the channels have the same layout and features. Better to just start from scratch.
Make sure that what you choose actually has (at least) the number of MIC inputs you need. Today fewer mics are generally needed, so fewer mic preamps. I have a Mackie 1202 that claims 12 inputs, but has only 4 mic inputs, and a Yamaha that has "16 inputs" but only 10 mic preamps. I agree with martyjoe that it's a good idea to have more than you think you need. You never know when one might go bad, or someone extra might show up. I was just going to suggest 10 or maybe 12 inputs though.
That said, brands I know include Mackie, Soundcraft, and Yamaha. Behringer is a good budget option, but I've not used it. Things like effects and probably USB will probably be included in that size mixer today, but you can just ignore them.
Good idea to work with a seller with good customer service if you're not too tech savvy.
Good luck. A good mixing console is a basic tool, and can be a long term investment.

Do you use passive PA speakers? If so the Behringer PMP 4000 is a great sounding mixer/power amp combo with 8 mic level inputs plus line level (stereo) ins. Setup is a breeze with no separate power amp to futz with.
On the minus side you only get 1 main out plus 1 monitor channel but we find that's perfectly fine for a contradance or small gig. Also useful for a dance setup is having per channel mutes, which a lot of inexpensive mixers don't have but this one does.
Look for an open box special from a reputable dealer.

Mackie - all the way. Been using their boards exclusively for 3 decades. Built for live gigging bands and I mean a lot of gigging. I've never had a single problem with them - no sliders not working or noisey, channels suddenly not working, just reliable mixing. We use a 24 Channel one now, but had a 4, a 8 and a 12 ch ones in the past. And reasonably priced too. Some brands are better off in the studio, Mackie is working band stuff.

In truth firstly I have had good luck with Behringer and Fishman. Friends have had good luck with Mackie. With gear it is always a gamble. If you need eight channels get sixteen is good advice. Having an extra chord or mic pays off. Good luck and play on.