Two hundred-fifty people were supposedly in that nightclub.
One hundred-fifty people escaped, approximately 50 people died, and 50 people were injured.
What hundreds of witnesses?
Just by the numbers you gave, there are 200 living witnesses who were in the nightclub when the shooting started.
If a significant number of them report actually seeing multiple shooters, then the story might be believable. I have only heard of one person saying this and so it seems more likely that person is mistaken and this whole idea is another Internet conspiracy theory.
As for how Mateen oould have shot so many people, one witness reported that he shot 30 people just in one location (they were crammed into a single bathroom stall). Apparently he had over three hours. And the police have admitted that some people could have been hit by police bullets.