Passing second argument to GNU parallel while feeding ls

The {#} replacement string should be exactly what you need here. For example, given

$ ls
 file1  'file2 with spaces'  'file3'$'\n''with'$'\n''newlines'   file4   file5

then

parallel --null echo {#} {} ::: *
1 file1
2 file2 with spaces
3 file3
with
newlines
4 file4
5 file5

or, if you have enough files to exceed the ARG_MAX limit you could use

printf '%s\0' * | parallel --null echo {#} {}

It is unclear to me what you want to do. Maybe one of these?

ls /folder/ | parallel -j20 ./command {1} {2} :::: - ::: {1..20}
ls /folder/ | parallel -j20 ./command {1} {2} :::: - :::+ {1..20}
ls /folder/ | parallel -j20 ./command {1} {#}
ls /folder/ | parallel -j20 ./command {1} {%}