Sonny,
I normally lightly tap the WSM's legs every 3-4 hours to knock down ash build up on the coals. If temps start to fall, I open the charcoal door and stoke the coals with stick (not too hard cause ash can fly everywhere including on your meat). I'll add coals if it doesn't look like there's enough to keep the heat up.
I'll add 20-30 coals depending on the situation but I don't want the temp to get too low. The amount of coal necessary depends on cooking temps and ambient temps. I use alot more coals in cold, rainy whether than in hot, sunny. With temps like we're having now (high 97 today), I can almost cook without coals.