You get +1 fervor for:
Each Stability point above 0.
100 Religious Unity.
100 Prestige.
Being at peace.
Base Value.
That's 7 possible fervor points a month, which lets you run 1 focus all the time and 40% (well, a little less since the activation costs 5 fervor as well) of the time another one simultaneously. So umh. You're statement isn't quite true.
I've just finished a
Reformed Ragusa game in 1.7. So:
1. Decent/great stability because I was a merchant republic and had plenty of monarch points. I actually boosted stability several times paying 300+ ADM just because my monarch points were overflowing. Even so, 3 stability is incredibly rare, so 1 point at least is always "missing". You probably have 2 stability 50% of the time (maybe?). So another point is more like 0.5 => an average of 1.5 fervor points generated from the 3 you mentioned.
2. Most of the time I was at peace (building everything I could). But in an expansionist game you won't be at peace much => 0.5 fervor points.
3. I had 100 religious unity from 1600 on, since I only wanted to conquer a set number of provinces so no expansion after I had those. Also during conquests religious unity is not 100% (unless you have Humanism, which you should - and in this case things are a lot easier) => 1 fervor point.
4. My prestige was not necessarily 100 because of the relatively few wars, but once I built the unique building, I think I was around 100 most of the time after 1700. Still, keeping high prestige is not hard when you fight a lot => 1 fervor point.
5. Base value => 1 fervor point.
Extra notes:
6. Switching also costs a bit, as you said.
7. Religious grants 0.25 fervor points, and I think you should get Religious ideas.
1.5 + 0.5 + 1 + 1 +1 + 0.25 = 5.25.
Yes, later on you will have one focus all the time (or close to it, more like 90%), while the second focus is much rarer, I'd say 20% or less.