I plan the arcs that I need to cover, and make sure to hit certain points of them, but I leave myself pretty fluid to be honest. I usually write a script for the chapter that's coming up next, but I tend to stray from it a lot, and rewrite parts of it as I go along. It's more there to help me remember the parts I need to hit. As I draw I'll come up with something better a lot of the times than what I had planned to begin with. It's really up to you.
In certain comics cases, 20 pages for a chapter actually feels way too short. I have a chapter that length, and it feels like barely anything happened in it. However, just like knowing when to break up a paragraph when writing, sometimes you just know when a chapter in a comic needs to be considered finished, or needs to keep going. I like to think of chapters as if they're mini-arcs. If that mini-arc isn't hitting the right point, I keep going.
Chapter lengths don't seem to matter all that much when you're doing digital comics I've noticed, but they do help with pacing to an extent. I see comics that have them, and comics that don't. It's all up to you. I personally like them, because they help me keep organized, and it's a great excuse to have a hiatus in between chapters and also show off a fancy drawing for a cover.
Sorry, I kinda started to ramble near the end, but basically all I'm saying is let your gut feeling tell you when the chapter has reached it's conclusion. Following a strict page limit can really kill pacing.