That means you're running with a limit of 63 local channels (+ console), and 64 globals. If you set_param 'XF', that will open the channel numbers to about 32k each (though documentation indicates a total limit of 500 open per session).
Beware if you have any old code that relies on 2- or 3-digit channel numbers, especially in strings. HFN with 'XF' will start at 32767 instead of 63. UNT will always count up from 1.