Pipeline code:

pipeline {
  agent { node {
    label 'my_computer'
   customWorkspace 'C:\my_folder'
  } }
 ....
}  

 
If I ran this pipeline concurrently on the same agent machine (same time):

Build#1 get C:\my_folder

Build#2 get C:\my_folder@2

Base on the documentation of customWorkspace, I would understand that both build#1 and build#2 would use C:\my_folder instead.