When I started my comic, I had 5 pages fully drawn as a buffer. I just had a rough outline of the next chapter and a rougher outline of the rest of the story.
That buffer didn't last too long, so after that I was pretty much finishing pages right before they went up. Working that way has advantages and disadvantages, but it did mean that a few times I had to skip a week when I wasn't able to finish the page, and put up some kind of filler thing instead.
When I came back after a long hiatus for family stuff, I had about 20 pages of buffer, because I wasn't sure I'd be able to work on the comic consistently, and I had done thumbnail sketches of the entirety of Chapter 4 (around 50 pages). That buffer of pages lasted about a year, so probably when I hit the end of this chapter I'll pause the comic for a few weeks to get a small buffer again.
It's definitely like.... a source of nerves, I guess, but I try not to let myself fret about it too much. If you miss a week, it's not the end of the world, as long as you communicate with your readers. Taking time off feels terrifying, like you're going to lose all your readers if you stop updating, but actually people are pretty cool with breaks as long as they know when you're planning to come back.