DOC - https://access.redhat.com/documentation/en-us/migration_toolkit_for_virtualization/2.6/html/installing_and_using_the_migration_toolkit_for_virtualization/migrating-vms-web-console_mtv

Section:  4.3. Configuring MTV settings
Parameter: Max concurrent virtual machine migrations

The new description should refer to the 2 source providers: VMware and RHV.
For VMware - The maximum number of VMs per ESXi host that can migrated simultaneously.
it means if there are 3 ESXi hosts in the vCenter and the parameter is set to "2" - The max VMs that can be migrated simultaneously are 6.

For RHV - The maximum number of VMs for RHV env that can migrated simultaneously.
also in case have multiple plans using the same RHV source provider.
it means if there are 3 migration plans (each plan with 10 VMs) for 1 RHV env and the parameter is set to "2" - The max VMs that can be migrated simultaneously are 2.