Ballistic missiles forms China's offensive firepower. The air force is really second to the ballistic missile force, for a variety of reasons. China's main concerns are at near-shore conflict zones, Taiwan, SCS, that could end up in high-intensity conflicts. Ballistic missiles are much more suited for high intensity conflicts, since they are hard to intercept etc. Plus ballistic missiles are cheaper to develop and technologically less intense than fighter aircraft which might not even survive in an high intensity conflict anyways. That's why you see countries like Iran and North Korea also focus on their missile force than airforce. Because missiles are cheaper to develop and harder to interecept than aircraft. Iran basically has a cobbled up airforce from the 70's, that relies on cannabilizing parts, while north korea has an outdated air-force from the 50's, that doesn't even have sufficient fuel.
The US focuses on their airforce more because they don't envision fighting an high intensity war where aircraft are not as useful. Instead they plan on small wars against weaker adversaries across the globe with no air defense systems, and the need to drop ordinance accurately and cheaply using airplanes is more important than survivability. That's why the US doesn't really focus all that much on their missile force (partly because they signed a treaty) but also because they don't really have a use for it, besides nuclear war.
The J-20 is more of a prestige project than anything that China really needs in any of the conflicts that could happen, to demonstrate that China has the capability to develop a 5th gen aircraft. Thats why there won't be hundreds of J-20's.
TLDR: Ballistic missiles have a higher return on investment than fighter/multirole aircraft when it comes to high intensity war. It is easier to shoot down a stealth jet, than it is to shoot down a ballistic missile.
