highest nice(1) -n increment value?

[Anton Shterenlikht]

It is not clear from the nice(1) man page, i.e.
for /usr/bin/nice, not a shell built-in nice,
what is the highest increment value nice will accept.

It seems it is limited to 20.
I tried

$ /usr/bin/nice -n 100 portmaster -a

But all processes spawned by the portmaster have
the nice value of only 20, as in:

  PID USERNAME    THR PRI NICE   SIZE    RES STATE    TIME    WCPU COMMAND
57586 root          1  52   20 13976K  4720K wait     0:00   0.39% sh
52729 root          1  40   20 13976K  4960K wait     0:02   0.00% sh
58239 root          1  92   20 35632K  8584K RUN      0:00   0.00% pkg
58237 root          1  52   20  9216K  1616K ppwait   0:00   0.00% make

The root shell priority was 0.
 
So is 20 the upper increment limit?

Thanks

Anton