Hi,
That is correct; one Nexus 5K, or a pair Nexus 5K in VPC can have up to 576 server ports. That number comes from 48 * 12 FEX, assuming each FEX has 48 server ports.
The ports between N5K and N2K are not server ports; they are special fex ports only used for connectivity between N2K and N5K. Depends on the oversubscription ratio you want to take, you can use 1 - 4 fex ports connecting N2K to N5K.
HTH,
Lei Tian