roughly, numbers are 8 bytes each, strings are about 32 bytes each, minimum. For your machine and a guess about your number of rows: 64GB/2B rows is 32 bytes per row. And, the row state information in the data table (selected, hidden, color, symbol, etc) uses 8 bytes per row. 3 columns of numbers, maybe.
It is possible to use short numeric integers of 1, 2, or 4 bytes in a data table; you'll have to enable them in preferences.
You probably have a page file of 64GB as well, which doubles the address space, but your data tables will be very sluggish once you start paging GBs of data.
Strings longer than a threshold of about 20 characters use additional memory.
JMP 12 has preferences that may help for importing the CSV. An extra pass is made to determine which columns can use the 1- 2- or 4-byte integer columns and which character columns have few enough distinct values for list check.
Craige