What is the Siege tank critical mass vs. Roaches?

Terran siege tanks can have a huge advantage against groups of units due to their AOE attack. But as they are very static, they are not very strong alone. There is a critical mass of tanks however, that allow them to beat opponents outnumbering them. Watch here the 1v5 siege tanks vs Zerg roaches and find out what the critical mass it that the tanks should reach.

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Make sure the tanks are all sieged before moving the roaches in. Initially, this was being done, but it stopped about halfway during testing, and i think it definitely would impact the results