The M series can sink and source a maximum of 16 mA on PFI<0..15>/P1/P2 lines. It can also sink and source a maximum of 24 mA on P0<0..31>. The counters share the PFI lines, so the specifications for the PFI lines are the same as the counter outputs. This information can be found in the Recommend Operation Conditions of the M Series Specs.
I hope this helps.
Jared A