Any Nexus 5000 can have a total of 12 FEX's connected to it physically.  If you have 12 FEX's in dual-homed mode, then that is the limit for both Nexus 5000's.  If you had 24 FEX's evenly distributed between the Nexus 5000's in straight-through mode, then that would be maximum in that configuration.  If you are mixing straight-through and dual-homed configurations, you would have to be within the 12 FEX per Nexus 5000 limit.

The 2148 is the first FEX, so yes it is the focus of the release notes.

The 2148 cannot have a local port-channel.  This is why you can only channel to a 2148 when using dual-homed (called Active-Active mode), one interface on each 2148, and it is tied together with a vPC configuration to make a port-channel.  The individual 2148's each only have one connection on them down to the server below.  The 2248 and 2232 do not have this restriction.

As of 4.2(1)N1(1), 576 refers to host interfaces.  If your host has virtual hosts, you just need to make sure you are within the limit of mac addresses in the system, which is 16,000 (13,800 unicast).

Regards,

John Gill

Reference:

configuration limits -

http://www.cisco.com/en/US/docs/switches/datacenter/nexus5000/sw/configuration_limits/limits_421/config_limits_4_2_1_chapter1.html