it depends if you want the ling rush to count, no having any back up strat, etc. if you want the ling rush to ping them and kill them with that one attack, you could go a 4 pool, and send your drones to attack later on, but thats avery risky move, of course, considering your starting position. if you want your ling rush to maim your opponent, to stop his economy, and attack with a larger force later, you could go 9 pool, then get speedlings, depending on if you think you can kill him with speedlings, you can keep going lings, like 6 more lings, but there have been instances in which i have seen people keep going lings the whole game, getting 2 more hatches, and going mass lings. (sometimes using the burrow strat)
do NOT go 8 pool, bad eco, bad speed.