Inside on text, $1- is a space separated list of token, $2- is going to be your list of comma separated value.
You do have to use $gettok, with $2- as the input then, and you want to generate a random number from 1 to the total number of token (item in your list), you can get the total number of item in the list using $numtok
44 is the ascii number for comma (32 for space) so $numtok($2-,44) is how many items there is in the list specified in !pick.
You can use $rand (or just $r()) to get a random number between two numbers: $rand(1,$numtok($2-,44)) is going to be a random number between 1 and the number of item in the list.
All in all, you get $gettok($2-,$rand(1,$numtok($2-,44)),44) to get a random item in the list

On *:TEXT:!pick *:#: {
msg $chan random item: $gettok($2-,$rand(1,$numtok($2-,44)),44)

The !pick * trigger could be improved as well; currently it triggers on "!pick " or anything not being a list of comma separated values.

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