20x would be a good choice for your 0.3mV 10 ohms cartridge, as well outlined previously. If you want a versatile SUT for other MC cartridges, then the Bob’s Devices Sky SUTs come with 2 gain settings. So either a Sky 30x/15x or a 40x/20x would be extremely versatile. I have a Sky 20/10 and Sky 40 myself; they’re exceedingly nice sounding boxes that work equally great with different cartridge makes.
You do have to watch out for MC cartridges that have a high coil impedance relative to a low output level. These can be trickier to match with a SUT since the higher ratios you’d normally use to bring up the low signal level will end up loading the coils down too heavily, which results in a significant loss of signal (i.e. you’re spinning your wheels or worse by going to higher SUT ratios). One example of this would be a Benz Ruby, with 38 ohm coils to 0.35 mV output (3 - 10 ohms is more typical at this output level). If you hook that Benz into a 40x you’ll lose over 7 dB of signal from loading down the coils (20dB * log(29 ohms / (29 + 38 ohms)), where 29 ohms comes from 47,000 / 40 / 40) - in effect, the net output is more like what you’d expect from a 15x SUT, and likely with other significant sonic aberrations to boot. However 40x on a 0.3 mV, 5 ohm Koetsu (which I’ve used) works just great. You could however counteract this issue with an MM stage that allows input loading settings above 47,000 ohms - which unfortunately is probably less common than it should be.
On the other side of the spectrum is the Air Tight / My Sonic Lab cartridges, which have a very low coil impedance relative to their output levels.
