bushpsu has it figured out. 1 AP or APA per battalion equivalent, so 3 per Regt, and an AK or AKA for the gear. Forming an amph TF always reserves some space for 1st phase (landing) supply. 3AP and 1AK was the standard early war USN Transport Squadron, rising to 3-4 APA and 1-2 AKA in '44/'45.
Additional supply in the assault TF is ok, but supply landed in the assault phase is lost (used up) at the end of the assault if you don't take the base/atoll/whatever in the first impulse. The supply auto-allocated to an amph TF is sufficient to support the assaulting units during the assault impulse, but you need back-up in case you don't win on day-1.
I like to do atoll/island assaults in pulses: assault TF, supply TF, follow-up (floating reserve) TF, supply TF, Infrastructure (Eng, AF units) TF, supply TF.
The sizes of the TFs will depend on what I'm assaulting and what it's defended by. It only takes one AK to supply a MarDiv for a day. Doesn't make sense to drop a gazillion tons of supply on the beach, when it will all be gone the next morning if you don't take the base. This is particularly important for atoll/small island assaults, where the troop stacking limit is 6,000.
Imagine hitting an atoll with an IJA Rgt (+) and an SNLF Bn in residence, with forts up the wazoo. That's going to take at least 2 Rgts (over 6,000 troops, right there) and probably more. So there goes your extra supply down the stacking limit violation rathole. I usually hit those places with 2 Marine Rgts and 2 Army Rgts, keeping a Rgt in floating reserve for each. I have several supply TFs lined up for daily supply service. Takes a while, but it worked for Spruance, so who am I to argue.