Is 64GB the max amount of memory for an Apple computer?

Depends what you mean by "current". The discontinued Xserve range of Apple servers maxed out at 96GB (although Apple only claimed 48GB).

More currently, the answer continues to be Yes and No.

• Yes, Apple only will support or sell 64 gig of RAM on the MacPro and iMac.

• No, the MacPro newer than 5,1 and most newer 27 inch iMac will accept and run quality memory that add up to 128 GB reliably.

• https://www.macmemory.com/

I would expect the next iMac Pro and Mac Pro to raise the ceiling for both the official limit and unnoficial limit.

https://www.apple.com/imac-pro/ According to the specs here for the new (not yet released) iMac Pro, you can order it from Apple with up to 128 GB of RAM.