When I ran my 10x10 legacy I had 14? households going at once. I had a few more than 10 because of the occasional set of twins that couldn't fit in the same house. But playing with that many households got a little hard because I don't turn aging off so I'd only really get to play each household for maybe a few in-game days, just long enough for the new YA to get married and get pregnant, and then it was on to the next.
If I turned aging off I could probably handle a lot more but I'm more of a legacy/generational style Simmer so that's not really my cup of tea.