2xmPCIe (m2), 1xmSATA (m2), 1xSATA.
There're also plenty of boards around Ryzen Embedded (V1605B/V1807B/etc) if you need more lanes. There's Udoo Bolt. Sapphire and iBASE also has several boards.
I'm not aware of any decent contenders with multiple m2 sockets based on ARM, if that's what you're specifically looking for. But you can always buy a cable or board that splits or bifurcates a single interface into multiple m2 slots. I haven't tried any of these myself but there are plenty of options on Ali, eBay, or directly from embedded/industrial vendors depending on your cost/reliability preferences.
I've spent quite some time looking around for the optimal low-footprint storage cluster options and eventually ended up with mITX form-factor after realizing I want plenty of RAM and need more cores per node to handle ZFS+gluster+encryption.
There are plenty of options if you don't, though!
I'd advice to check out the APU's, either way. They have served me very well until I started upgrading and have now been repurposed for other use.