20 for infantry, 12 for cavalry sound about right, but you can go up and down.
Kudos taking a chance on Runestar D12 and getting a game in. Really digging your miniature collection, I do a similar aesthetic

I usually run shieldwalls at 25 infantry. If you really want them to hold the line give them full command and a hero for that vital +1 morale bonus.
For shock infantry (great weapons, etc) units of 10 can be nasty as flankers (remember that enemies have to fight forward before they can fight sideways). Fighting in two ranks every turn, great weapons can devastate even the most powerful foe.
For cavalry it depends - a small unit (8-12) can outflank an enemy you have pinned down with your infantry, but a larger unit (16-20) with full command can really dominate a battleline and can even take a few charges. Generally speaking head-on cavalry charges will get bogged down; they do much better with a support unit flanking the enemy and forcing morale tests. In one game a humble unit of ten spearmen flanked an enemy block, forced them to test, pushed them back and allowed my shock cavalry to charge again and get strength bonuses.
Runestar D12 is all about grinding the enemy down - once you fall below one rank you are effectively toast!
p.s. The Rune Magick system is really the best thing about the game! Definitely where most of the innovation can be found
