Synology and QNAP are both very good choices - I'm in the QNAP camp, but appreciate the capabilities of Synology on their NAS boxes...
Asustor is an up and comer... but also consider Netgear's ReadyNAS platforms, as they have history over years like QNAP/Synology...
ARM-based, they're all pretty close these days - x86 basically has two levels, the small cores (Bonnell for older x86, and Silvermont's for newer), and the big cores...
1/2/4 or more bays - depends on your needs and requirements - it's just more storage and flexibility with the disks - 2 bay NAS on Intel x86 J1800 (e.g. TS-251 in QNAP speak) is at a decent price/performance point...