Any arguments specified on the command line are given to utility upon each invocation, followed by some number of the arguments read from the standard input of xargs. The utility is repeatedly executed until standard input is exhausted.

and

-P maxprocs Parallel mode: run at most maxprocs invocations of utility at once.

The way I interpret that is that you could run xargs in parallel mode, but by default the "utility is repeatedly executed" in the same process.