Depending on the submarine design and the miniaturization of the missiles it uses they might be able to do with less submarines.
Current Type 094 submarines carry 12x JL-2 SLBMs. Compare that with Russian, French, or British submarines which can carry 16x SLBMs.
That's like 33% more missiles per submarine so to carry the same amount of missiles you would need 8 submarines instead of 12.
Current US SSBNs built in the late Cold War are even larger but that does not seem cost effective to me.
Putting more missiles/warheads on fewer boats is undoubtedly more efficient than fewer missiles/warheads on more boats, but such efficiency is a relatively low priority in creating an effective Continuous At-Sea Deterrent. The greater priority is ensuring that the SSBN force is survivable in the first place. Missile capabilities contribute to that, and the technical characteristics of the boats are obviously of great significance. But numbers are almost as important. More boats translates to more at sea at any given point, and greater surge capacity, and this matters because USN, JMSDF, etc. have considerable but nonetheless finite resources that they can devote to tracking those SSBNs. If USN is confident that it can track and neutralise China's small SSBN force, the nuclear deterrent is compromised and this would allow the United States to take actions against China that it might otherwise be deterred from.
All else being equal, an SSBN force of 8 boats with 12 missiles each is a more survivable and therefore more credible deterrent than a force of 4 boats with 24 missiles each.
I think the land attack or anti-surface role will likely be related to attack submarines with smaller displacement. China has enough surface ships that large missile boats aren't as necessary and they just lack nuclear submarine hulls to use for such niche applications.
Surface ships (or land-based rockets/missiles) cannot substitute for the SSGN in the anti-carrier role because their locations are likely to be known to the carrier battle group well in advance. This knowledge, coupled with the high speed of the carrier group, means that it is the carrier group that dictates the engagement, i.e. if, when and how it is to occur. If a surface action group engages a carrier group, it is because the carrier group allowed it to happen, which bodes ill for its chances of success. The SSGN changes all this. It can approach within launch range with a good prospect of not being detected. It can approach and attack from unexpected directions. In short, within limits an SSGN can dictate the engagement.
Yes, China lacks nuclear submarine hulls. A modest expansion will not suffice. Massive investment is required in SSNs, SSGNs, and SSBNs. Nuclear submarines are ultimately far more important to China's security than aircraft carriers.