"Number of plan statistics captured" tells you how many executions Oracle recorded information about the plan.
It uses these to populate the "Rows (1st) Rows (avg) Rows (max)" columns in the row source operation. So
Rows (1st) Rows (avg) Rows (max) Row Source Operation
---------- ---------- ---------- ---------------------------------------------------
1 1 1 TABLE ACCESS BY USER ROWID T1 (cr=1 pr=0 pw=0 time=25 us cost=1 size=2551 card=1)
Tells you that of the 3,344 plans captured:
The first processed 1 row
The mean of these processed 1 row
The most any of these executions ever processed was 1 row.
In short, this plan always processed 1 row ;)
"library cache: mutex X" is unrelated to this. This is a wait event.
library cache: mutex X 3 0.00 0.00
Tell you that you waited 3 times for this. For a total of zero seconds ;)
So nothing to worry about in this case. You can find more about wait events by reading:
http://docs.oracle.com/database/121/TGDBA/pfgrf_instance_tune.htm#TGDBA95374