You need to read the entire description of Sustained Assault, not just the beginning. Eldar Codex, Page 38, under the title "Sustained Assault"
"The Exarch keeps his wings in constant motion, darting from one foe to the next in a continuous attack. For each close combat hit he inflicts, he may make an extra attack. If an extra attack hits, then this allows him a further attack. Keep rolling to hit until the Exarch misses. Resolve to wound rolls and armor saves for each hit as normal."
This is pretty clear. You keep rolling until you miss with all of your attacks. That means if 3 attacks hit the first time, you roll 3 more dice. If only 2 attacks hit the second time, you roll 2 more dice. If one attack hits the third time, you roll one more dice. If that one attack hits again, roll again. If you roll really well, you can kill an entire squad in one round of combat.
Hope this clears up the confusion!
Blackfel