What does "Hard Memory Controller" number mean?

Yes, you could potentially create up to 6 DDR4 memory interfaces though the interface widths would be limited by pin availability. Hard memory controllers are available in all the I/O banks of the device to provide flexibility for implementation. See this online training for an introduction to the memory interface architecture for these devices:

https://www.intel.com/content/www/us/en/programmable/support/training/course/omem1121.html

#iwork4intel