What amp rating of 24vac transformer for LM317T?
You first have to figure out the maximum output DC current of your power supply. This can be determined by your load, by the maximum current handling capacity of your regulator or diodes, or (most likely the case) by the maximum power dissipation at your regulator.

Once you know the maximum DC current, you have to determine what will be the RMS current at the transformer's secondary windings. This is actually more complicated than what it seems  because the current waveform at the transformer is not trivial - the current only flows through the windings of the transformer when the voltage is higher than the capacitor voltage plus the voltage drop across the diodes.

I'll give you two choices at this point. If you're really enthusiastic about it and want to know more than 99% of the linear supply designers, I encourage you to simulate the circuit and calculate the worst case RMS current (square root of average squared current) of the transformer for your case. If you just want a quick answer, a good rule of thumb to use for the rms current at the secondary is (see reference below):

for center tap secondary with 2 diodes: \$I_{rms} = 1.2  I_{DC}\$

for 2-wire secondary with 4 diodes: \$I_{rms} = 1.8  I_{DC}\$

Keep in mind that the current at the primary of the transformer will be the current at the secondary multiplied by the inverse of the voltage ratio of your transformer (well, there is also magnetization current at play, but we don't need to get there). For example, if the primary voltage is 10x the secondary voltage, then the primary current will be 1/10 of the secondary current.

P.S.: I recommend you take a look at https://www.smcelectronics.com/DOWNLOADS/1980-VOLTREG.PDF, chapter 8 starting at page 74. Don't be impressed by the age of the document: linear power supply design is an old and almost forgotten art and therefore relies on almost forgotten references. :^)