At the absolute minimum imo the H-20 should at least match B-21 numbers (at least 145 planned so far?) and then at least a bit over, so probably lower hundreds for H-20 alongside mid-hundreds for GJ-X, plus probably high-tens or even low hundreds for WZ-X. Since such a program will likely be spread over a decade/decades, this is quite conservative.
They have a different mission sets and requirements.
The B-21 is supposed to conduct attacks on mainland China, which has tens/hundreds of thousands of worthwhile aimpoints.
So the US could usefully use many hundreds of bombers.
Of course, that assumes there are bases available and the bombers aren't destroyed on the ground.
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In comparison, let's work with the assumption that for the Chinese:
1. the GJ-X is primarily for the 2IC
2. the H-20 is for the 3IC and beyond
It just seems to me that even 100 of each type is overkill in terms of direct attack mission requirements.
After all, it is only a handful of isolated airbases, naval bases and aircraft carriers.
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You could make a case for large numbers of GJ-X stealth tankers to support the H-20s for long distance missions.. But you very quickly come up against physics, with the tankers burning much of the fuel just to reach the refuelling locations. That applies even at a distance of only 2000km from mainland China. So you conceivably would need a lot of stealth tankers for every bomber.
But my guess is that in 5-10 years time (which is when a new stealth tanker could come into service), the Chinese Navy and Air Force should be able to provide air superiority to a distance of 2000km offshore anyway. So they can just use Y-20 tankers, which already exist, and are a significantly cheaper option.
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NB. The US has the same problem if they want to operate bombers from Hawaii to China.
But it's arguably worse given that the H-20 is likely being designed for more range (and longer ranged weapons) than the B-21
The US would need tankers refuelling tankers, in order to reach China.
In comparison, the H-20 might not need any tankers, if they are using 2000km-3000km range large hypersonic missiles.