Short answer: as many as your plan needs, decided in advance, and they should not count against you. The number matters far less than whether you chose it deliberately or just failed to show up.
A rest day and a missed day are different things
This is the distinction most tracking gets wrong, and it does real damage.
A rest day is part of the plan. You decided, while thinking clearly, that Saturday is not a training day. Nothing is owed on Saturday. There is nothing to make up.
A missed day is a day you intended to do the thing and did not.
If your tracker treats those the same, it is lying to you twice a week. It marks you down for a decision you made on purpose, and after a few weeks of being marked down for resting, most people do one of two things: they train on their rest days, which is how people get hurt, or they stop trusting the tracker, which is how people stop tracking.
How many, in practice
It depends entirely on what the daily thing is, and this is not a question a habit app can answer for you.
Strength training is the clearest case. Muscle is built during recovery, not during the session, so rest days are not time off from the plan, they are part of it. Most general strength programmes are built around three to five sessions a week.
Something cognitive, like a language or an instrument, generally rewards frequency over intensity. Daily is often genuinely better here, and the rest day you need is a shorter session rather than a missed one.
Something purely administrative, like ten dollars a day into savings, may need no rest days at all, because it costs nothing to do.
If you are not sure, five on and two off is a defensible starting point for anything physical, and you can tighten it later. What you should not do is start with seven and quietly drift to five, because then the two are missed days rather than rest days, and the whole record reads as failure.
Decide the pattern before you start
The reason to fix it in advance is that a rest day you choose on the day is indistinguishable, to your own memory, from a day you skipped. Two weeks later you cannot tell which was which, and the doubt is corrosive.
A repeating weekly pattern is usually better than a list of dates. It matches how people actually live, it survives you changing the length of the journey, and it does not require you to plan ninety individual days in advance.
What happens when you take one
Nothing should happen. That is the point. No prompt, no notification, no gap in the record, nothing to catch up on. A rest day that generates a reminder is not a rest day.
And the length of the goal should be counted in efforts, not in calendar days. Thirty check-ins with weekends off is still thirty check-ins. It takes about six weeks instead of four, and that is simply the truth about the plan you chose rather than a shortfall.
How Reveal the Dream does it
Rest days are a Premium feature. You choose which weekdays ask for a check-in, and the days you turn off are genuinely off: nothing to do, nothing to miss, nothing uncovered. The journey still asks for the same number of check-ins, so a 30 day goal is still thirty pieces of your photograph. Only the dates move, and the app tells you how many weeks that works out as before you commit.
The free version checks in every day, which is what it has always done. You can try the picker on the homepage and see the sentence change as you switch days off, or read what happens on a day you genuinely miss, which is a different problem with a different answer.