The Wall Street Journal reports (via 9to5Mac, if you don't subscribe to WSJ) that there are about 33 million acitve Apple TV+ subscribers. That makes is half as big as Netflix, and about 9 million smaller than Amazon Prime, but larger than Hulu and Disney+ (all in the US only, not counting international at all).
These are estimates, so they're up for debate, and Benjamin Mayo suggests the number is likely closer to 10-15 million users.
As 9to5Mac points out, rivals will point out that everyone who bought an iPhone, iPad, Apple TV, or Mac got one year for free, so while most of those Netflix, Hulu, and Disney+ users are actively paying for the service, the number of Apple TV+ subscribers who have spent a cent on it is likely much, much lower.
But this all gets to the point that comparing these things is really difficult because it's not an apples-to-apples comparison. Apple TV+ and Amazing Prime Video customers may not be spending a cent on the services, but they use them because they get them "for free" for buying something else. There are also deals and packages that get you other services like Disney+ for free as well.
These are analysts' predictions, not hard numbers, nor are they neccessarily indicators of how much people enjoy these services. They're interesting, and can give you a ballpark idea of how things are going, but don't get too married to any of these numbers.