iRule to forward requests to pool with most active members

Depending on how you define "the most available members", here's one you can try:

when CLIENT_ACCEPTED {
    if { [active_members pool_1] >= [active_members pool_2] } {
        pool pool_1
    } else {
        pool pool_2
    }
}

.

Try this code

when CLIENT_ACCEPTED {
    pool [expr { [active_members pool_1] >= [active_members pool_2] ? pool_1 : pool_2}]
}