Is 64GB the max amount of memory for an Apple computer? I was looking at a comparison between 2017 Apple computer offerings and the highest end system appears to be the Mac Pro, a tower-type computer 6 inches in diameter and 9 inches high. This system is described as expandable in memory up to 64 gigabytes. So, is that the top-end limit on memory for current Apple computers?
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.