Aaand because I've only just noticed the question (Alianora, pay more attention in future!):
The minimum number of races in a championship for it to be classed as a valid international FIA championship is 8. That's in the regulations. So from the FIA's perspective, F1 could start in Monza, not reschedule anything, and still be hunky-dory. Beyond that is a problem. Multiple races per venue is allowed, but only if the FIA agreed to that in advance. It shouldn't be an issue, considering the FIA and Liberty have full authority to sculpt the calendar without team interference. However, for reasons given below, just leaving it at 7 is not an option. It would lose the FIA calendar status, teams would not be paid (except for the CCB payments) and it would risk the series not being issued a licence for 2021. (If the FIA was annoyed enough, it could do what happened in 1953 and re-label F2 as F1, opening out the regulations as teams appeared ready. In the modern take, F3 would be relabelled F2, and F4 teams invited to join the new F3. Unless they thought Liberty was at fault, in which case they could refuse to issue any of F1-F3 licences until a more appropriate promoter was found). F1 would prefer not to go through this outcome.
The minimum number of F1 races Liberty needs to make its contracts is... ...different. Nobody's 100% confirmed it, but Dieter Rencken of RaceFans appears to have evidence indicating the TV broadcasters only have to pay Liberty if there are 15 rounds. It's not clear whether there can be two rounds at the same venue and still count (the answer may even be different for different broadcasters). If so, the season can start as late as Singapore and provided all but one of those places runs two races in each weekend, everything may also be fine from TV perspective. If it doesn't, there's a problem. TV is about half of Liberty's income from F1 (tracks are perhaps 40-45%, and the remainder is from F1TV and other in-house initiatives).
Venues pay no matter how many races there are, provided their venue is one of them and the venue's name is not Monaco*. However, they don't pay if it is cancelled by anyone other than themselves. While it should be hoped that an equitable pay system is in place, the Melbourne farce suggests it is not. Also, venues only pay single rate whether 1 race is held there per season, or 21. This is why repeat venues, and indeed multiple races per venue, is of limited help to Liberty.
The venue revenue stream and TV revenue stream are similarly important. Liberty's ideal is 1 venue per season, and as many venues as possible in a season. Interestingly, the time element is the least sensitive part for Liberty; any extra time it can get people to sign up for without missing any avoidable races (thus losing out on venue income) benefits the "miscellaneous" income because F1TV is monthly. The tension points for that are subscriber patience (the non-live bit generates some money, but not everyone will use it for the whole spring interregnum) and TV broadcaster credulity.
I would seriously consider putting in a "superseason". Not necessarily a full 18-month one like WEC did, but also not necessarily just 12 months. If the world opens up again at the end of June (which I think is the earliest everyone will get to do it, as opposed to just some people), I'd schedule like this:
August: Hungary/Canada or France (if Liberty must break the summer holiday)/Belgium
December: Bahrain/Abu Dhabi (move slot by a fortnight)
January: Vietnam (or Vietnam/China if emergency slot needed)
February: China (or dates to move autumn "second wave" postponements to)
May: The Netherlands/Spain/Monaco
I would then have an abbreviated 2021 season, that was just the remaining rounds as per the original 2020 schedule. Baku would be the opening round and Abu Dhabi the season-ender. Two of Bahrain, China or Vietnam with the Japan and Abu Dhabi rounds (and both pairings are needed to hit the 15 required for TV).
If the definition of "season" does not allow for this, then simply doing a WEC-style superseason, with some events going twice, is perfectly fine.
* - This is part of the reason Monaco is cancelled instead of postponed. Liberty is required to attempt rescheduling rounds unless authorised not to do so by the promoter, otherwise it has to pay the costs the race has accumulated so far. If the promoter gives that authorisation, it must pay those costs, plus the hosting fee. Monaco pays no hosting fees, so the promoter's not going to be fussed about paying an extra £0 to cancel first.
The other part is that Monaco knew in advance that this was the only weekend of the year it could get planning permission to race (believe it or not, some people who live in Monaco object to the disruption even when it runs conventionally, even though it's been like that since before they were born/moved in). It's not like Azerbaijan, where there are a few candidate weekends that the government could use because it doesn't have to worry about "planning permission" in the same way. Once it was established the original date was a no-go for Monaco, delay could only lead to more set-up/take-down expense.