Theoretically the US and Japan will be able to muster in excess of 1000 missiles from a combination of bombers, naval aviation, shore based missiles like the modified Japanese Type 12s, surface fleet and subs.
In reality it'll be a lot more complex operation than that to pull off and a lot of things have to go right.
In a hypothetical "Allied" first strike, I imagine the most likely first target along with the PLA amphibious fleet will be China's carrier group(s) patrolling on the east side of Taiwan. With the J-15s and J-35s patrolling the waters of the Philippines Sea, the bombers from Guam will be impeded to get near Taiwan. So the first task of the bomber group will be to wipeout the Chinese CSG. However, they'll be releasing their weapons within combat radius of the J-15, and possibly also HHQ-9s depending on the CSG location. This means they'll need fighter support from naval aviation from CSG to fend J-15s off, so the Americans will station a CSG or more in the Philippines Sea ahead of time to coordinate with the bombers. China may have J-20 long range CAP here as well.
Then it'll morph into a duel between CSGs + bombers. PLA will rush H-6K/J/Ns and the DF-17, DF-21D and DF-26 brigades, while the Americans rush their bombers and additional subs. The Americans would likely need to sacrifice their prized CSG and some bombers but if they succeed in destroying the PLAN CSG, the way will be clearer to quickly launch a 2nd wave of bombers and subs to hit the rest of the PLAN fleet near Taiwan.....at least in theory. DF-26 will probably have crippled Guam by then. So I think the 2nd wave may have to come from B-52s from faraway bases like Pearl.
Simultaneously, while the CSG battle is going on I imagine the Americans and Japanese will try to launch as many missiles as possible from the 1IC into the vicinity of Taiwan, as these will be in a use it or lose it scenario as China unleashes a huge missile counterattack wave. Whether this will actually be able to critically cripple the PLAN amphibious operation is another story.