If it's a consumable, they should have tested at least one hull to destruction, just so they could be sure of it's failure modes.

Based on what I've seen, it doesn't sound like OceanGate had decided on a fixed number of dives. Instead they were planning to continually monitor it (x-ray scans after every dive and real-time acoustic monitoring during the dive) with the assumption that they would get some warning before it failed.