Which CPUs does the C9Z390-CG support?

It uses the LGA1151 socket with the Z390 chipset, so it takes 8th and 9th Gen Intel Core i9, i7, i5 and i3 processors, plus Pentium and Celeron chips on that socket. Older 6th and 7th Gen CPUs are not supported.

How much memory can it hold?

Four DDR4 DIMM slots, which supports up to 128GB with 32GB modules. Check Supermicro's qualified memory list before buying large or high-speed kits.

No. Networking is wired Ethernet only, so add a PCIe Wi-Fi card or a USB adapter if you need wireless.

Is it good for overclocking?

It runs unlocked K-series chips, but the BIOS and power delivery are aimed at stability, not tuning. Buy a consumer Z390 board if overclocking is a priority.

What form factor is it?

Standard ATX, so it fits most mid-tower and full-tower cases. Check clearance for your CPU cooler and graphics card.

Is it worth it over a mainstream Z390 board?

If you want Supermicro's build quality and validation for a work machine, yes. If you want Wi-Fi, RGB, more ports or easy overclocking, a consumer board gives more for the same role.