Setting MAX memory for SQL servers depending upon the model of CPU
The max memory setting is how much RAM SQL will use. It will usually not use more then the max memory set. (Following Shanky's comment, see below for the exception to that rule): Quoting from  Memory Management Architecture Guide




  
  Memory allocation requests made directly to Windows: These include Windows heap usage and direct virtual allocations made by modules that
  are loaded into the SQL Server process. Examples of such memory
  allocation requests include allocations from extended stored procedure
  DLLs, objects that are created by using Automation procedures (sp_OA
  calls), and allocations from linked server providers.
  


Now if your hardware has 1 TB but only 768Gb are recognize in the OS, setting up the max memory to higher then 768Gb will allow SQL to use all available RAM (all the RAM detected by the OS) and this could cause potential issue with the OS as it will have to do memory pressure on SQL to claim it back.

Set the SQL MaxMemory setting depending on how much RAM is seen by the OS and it should be all good. (Leaving at least 4 Gb for the OS)

P.s. Do not set min memory setting the the same value as the max memory. Set it lower as SQL will not release RAM under the min value when there is memory pressure and this could lead to an OS crash.