"Sentry Busters will start spawning once an engineer's sentry has reached 15 kills or 3000 damage (whichever comes first).

Interestingly, these thresholds decrease when multiple engineers are on the team. The game adjusts the thresholds based on the red team's total number of sentries that are currently built in the level (excluding disposable sentries but including Gunslinger mini-sentries).

It's a roughly linear decrease, and with 6+ sentries, the thresholds bottom out at half of their normal values. The kill threshold is rounded down (since fractional kills are fairly nonsensical), whereas the damage threshold is effectively not rounded.

Having multiple engies on a team already means that there will be chaos due to multiple Sentry Busters spawning for different engies' sentries; but on top of that, each engie will have sentry busters spawning for them sooner than they would have otherwise, due to the mere presence of their teammates' sentries."