Tente isso como um ponto de partida (não testado)
#! /bin/bash
# LINKS is a list of space separated links use text below, otherwise
# replace first uncommented line below with
# for each in 'cat $LINKS'
for each in $LINKS
do
count='ps waux | grep "curl" | wc -l'
until [ $count -gt 4 ]
do
count='ps | grep "curl" | wc -l'
sleep 1
done
curl $each &
fi
done