I don't think anyone knows for sure. Certainly there have been no test results released to the public.
The best one can do is assume a range of durations needed for one interception, in seconds. And then calculate the best and worst case scenarios, given that the effective range of the weapon is known, the missile speed is known and the re-targeting time allotment is probably not going to be more than a few seconds, again depending on the target position.
11 seconds needed for the missile to close in from 3000 m to 0 m. 1-3 seconds of fire assumed per target. Initial muzzle speed 1200 m/s? But average speed over 3 km surely less. (1200 m/s is estimate) So likely at least 4 seconds of dwell time per target, possibly 6 seconds. With that time getting lower as the missile is closer. If retargeting to another missile takes a second or two, it doesn't seem likely than more than 2, and likely not more than 3 could be engaged. With possibly not all 3 actually taken down in time. I'd say that 4 would also be the absolute theoretical limit, as well.
Unless the missiles are staggered, so they don't impact the target at the same time, and unless they're flying one right next to/behind another.