I'd ballpark it as "several hundred." (Edit to add: this was a guesstimate based on "having a sense for this sort of thing is a key job skill for me." I then went looking to firm up the guesstimate. Data points in favor: 400 people match a search for "Airbnb engineer" on LinkedIn. A recent post shows a photo of their data science team with ~30 people on it; on previous experience with software companies, I'd have difficulty thinking there aren't at least 5+ much larger teams.)

More broadly, I think people consistently underestimate how much you can get done with a very small team at a startup and, simultaneously, underestimate how many engineers it is possible to throw at a single product.