How many batteries do you usually use to shoot a full wedding with the SONY a7r iii?
There is no sensible answer to your question. Different photographers shoot different amounts to cover different weddings. (There is no standard number of images for wedding coverage.)
Expect to use 2 - 3 times as many batteries as you would use shooting the same number of images with a modern dSLR (on average). (The CIPA testing results using EVF on the A7R III is 530 shots. The Canon 1D X Mark II is CIPA rated at 1,210 shots, The Nikon D5 is CIPA rated at 3,780 shots.)
Well, I may need 4 of them, I think.... maybe 5. Costs add up...