My lvl 2 fortress (capital) has 21! militia.  This is turn 97, and I've saved/quit LH/restarted a few times.

Experimenting, I saved (without doing anything -- didn't end turn, move units, etc.), quit LH, restarted and loaded the save game, and checked -- 23 militia.

I again saved (without doing anything -- didn't end turn, move units, etc.), quit LH, restared and again loaded the same save game, and...

Now 25 militia.

So, quitting then restarting appears to be incrementing 2 militia each time.

(wondering what would happen if I was attacked and there were more militia than tiles to place them...).

In this game I have a town and a conclave, and both have the normal number of city defenders.

Game settings are huge world, ridiculous world and AI Sovs, my custom kingdom vs. the 4 default empires, no mods.