Whoa, let's hold up there for a moment.
At the point of the war of attrition, in which the US would consider putting any substantial US personnel onto Taiwan, all of the below would have happened:
- 2-3 years of a generalized war of attrition between China and the US would have occurred, whereupon China would have essentially lost the capability to project any air and sea power to project over Taiwan in general
- PLA forces on Taiwan (which I generously and optimistically describe as a PLA group army that the PLA have managed to land before the US made its intervention into the conflict), would have had their sealift resupply crippled and most of their airlift resupply greatly hindered for the last year and a half, making their forces poorly supplied at best by the time a US invasion of Taiwan occurred.
- The US would have conducted at least 1 year of bombing of the PLA forces on Taiwan, which would reduce the PLA on Taiwan to essentially a force of light infantry without any meaningful AFV, artillery or air defenses, and with manpower significantly depleted as well.
- The US would have conducted airdrops of materiel to ROC insurgents and coordinated with them throughout all this, to enable them to designate PLA formations for airstrikes over the prior past year and also conducted harrassing attacks against the PLA, all of this in an island territory where the civilian population is at best distasteful of the PLA, and at worst actively hostile to the PLA.
Of a 40,000 strong Group Army that the PLA was able to deliver, I would be pleasantly surprised if half of the PLA's manpower and one fifth of the heavy equipment remained, by the time the US considered deploying boots on the ground onto Taiwan island.
To robustly defeat such a force, I imagine the US might require the equivalent of a US Marine Expeditionary Force if they were forced to fight the remaining PLA forces head on, unsupported.
However, the US would of course have air and sea control around the whole of Taiwan and able to provide air support with a handful of CSGs and a handful of LHD/LHAs (the latter as part of the amphibious assault fleet), and PLA forces on the island would be struck by ROC insurgents to coincide with any US military landings, and the PLA forces would essentially by now have been reduced to some 20,000+ light infantry lacking heavy fires support (and obviously no air cover at all).
Given that, I would be surprised if the US would need more than a US Marine Expeditionary Brigade in terms of landed manpower, which requires 5 LHD/As, 5 LPDs and 5 LSDs to deliver. With support of Afloat Forward Staging Bases and a couple of CSGs (and the regional air bases that the US has), I absolutely expect the US to be capable of defeating a moribund, harried, mostly light infantry force of some 20,000+ PLA light infantry on Taiwan.