I am far, far from an expert but I am using a Yamaha mixer which is great. My research showed a lot of people having problems with bargain brand mixers--poor reliability, hum and crackle, etc.
I chose the 10 channel Yamaha with USB:
https://smile.amazon.com/Yamaha-MG10XU-10-Input-Stereo-Effects/dp/B00IBIVL42
It isn't cheap at $210 but it is well built and has been a real pleasure to use. I have enough inputs for anything, it supports lots of cable formats, it has compression on a couple of inputs, and the USB output makes it really easy to use with various PC apps.
The effects aren't all that, but a little vocal echo can be super nice at times.
You may be able to get something just as good for $29.99--like I said, I am not an expert, or even a journeyman, or even much of a hobbyist. But when I looked in to this, the Yamaha seemed like a very safe choice and it hasn't let me down yet.
Good luck!
You may want to consider a mixer with USB. Assuming you are using karaoke software on a PC, you can get the music into the mixer via USB. I find that nicer than messing around with audio drivers and those crummy 1/8" plugs on the computer.
This Yamaha has plenty of inputs, USB, compression on 2 of the mics, and some decent FX.
https://smile.amazon.com/Yamaha-MG10XU-10-Input-Stereo-Effects/dp/B00IBIVL42?sa-no-redirect=1
I am no pro, or even advanced audio hobbyist, but this Yamaha mixer has been super.
Supposedly this mixer has really good effects for singers, but it also costs a lot more:
https://www.sweetwater.com/store/detail/ZED10FX--allen-and-heath-zed-10fx-mixer-with-usb-and-effects
There are even mixers with autotune built in but I don't know anything about them.
Yamaha MG10. $199.
1/4" and XLR 3-pin ins and outs.. EQ (bass/treble control) on channels. Has a few extra channels more than you need at this moment, because you WILL eventually need "1 more channel"... You could go smaller/cheaper on the mixer, but you'll likely outgrow it quickly. Stay with Yamaha, Mackie. Don't get 'Harbinger' (from Guitar Center), I'd stay away from Behringer's cheap stuff as well.
https://www.amazon.com/Yamaha-MG10-10-Input-Stereo-Mixer/dp/B00I2J4TWG
If anyone will be singing in the mic, now or in the future, the FX model with reverb is only $30 more.
https://www.amazon.com/Yamaha-MG10-10-Input-Stereo-Mixer/dp/B00IBIVL42?
You'll need a Lightning to 1/8" TRS female adapter, and a 1/8" TRS male to dual 1/4 TS male cable. 1/4" ends go into channels 3/4 L+R. Cables to house speakers come from "Stereo Out" in blue box on mixer. Done.
https://www.amazon.com/Apple-Lightning-Headphone-Jack-Adapter/dp/B01LXJFMGF
https://www.amazon.com/Cable-Matters-3-5mm-6-35mm-Breakout/dp/B07PHVMTNT
OK I'm making a few assumptions here: first that your audience is about 50 people. And your participants will sit at a wide table up at the front of the room, facing the audience. And your audience members will walk up to a mic on a stand to ask questions.
can't I use an XLR to 1/4" headphones jack adapter to use the other inputs on the scarlett?
No. For 4 mics you need a mixer with 4 actual balanced XLR mic inputs with phantom power for each one. The 1/4-inch inputs on the Scarlett are line level inputs and won't accommodate a microphone signal which is much weaker.
Also, you need to create two different audio "mixes" -- Mix A consists of the local mics, which will connect via USB to the laptop to be sent to the people on the Zoom call. Mix B consists of the local mics PLUS the receive audio from Zoom call, which will connect to the powered speaker so the local participants and the audience can hear them. Depending on the room size / audience size / seating layout, one speaker may not be enough. You might need one aimed at the "panelists" and one aimed at the audience. The Mackie speaker is fine, but probably bigger/more powerful than you need unless you're expecting 100 people. Something like this would be ideal for the monitor for the panelists. https://www.behringer.com/product.html?modelCode=P0E2W
You need a mixer that has what's called an "Aux Send" which allows you to route some channels to it in addition to the main output. There are many options, but https://www.amazon.com/Yamaha-MG10XU-10-Input-Stereo-Effects/dp/B00IBIVL42?th=1 is a popular and cost-effective one. It also has a built-in USB connection AND the ability to route the return audio from the laptop/Zoom call to one of the input channels so it's easy to adjust the level.
By the way, you mention 2-4 speakers PLUS an audience mic which means you need 5 mic inputs. Mixers usually have an even number of inputs, so you may need one with 6 or 8 XLR mic inputs.
Also think about how you will mount the MX412D for the audience QA. I would suggest a plain old SM58 on a mic stand. There's no button so you control it from the mixer. You could get a wireless one if you want.
Also with this setup, you are relying on the echo canceller in the Zoom app -- which is designed to cancel the echo from one mic and one speaker whose positions do not change because they're on your desk in a small, quiet room. You will be selecting different mics in different parts of a possibly large and noisy room with a live audience (they're going to be nice and quiet and polite, right?) that requires louder sound levels, so the echo problem could be more than the Zoom app can deal with.
You can deal with this in two ways: #1 is you (the system operator) only turn up the Zoom participant when it's actually their turn to talk. Your meeting scenario might or might not permit this. If it's a casual back-and-forth discussion where the Zoom people can chime in whenever they want, you may need option #2: an external audio DSP with a dedicated echo canceller for each microphone. Shure makes hardware and software versions but they require networked Dante microphones, which adds another layer of equipment here that you probably don't want to deal with and pay for.
The MXCW system is a great option. And the nice thing is you can scale it up (to 125 people) and it's wireless so setup and teardown take only minutes, and there are no cables for people to trip over.
FYI our Applications Engineering team would be happy to work out all the details if you have questions. www.shure.com/contact