Throughout The Flash we have seen that during the moment Barry's mother dies there is one Barry that is fighting with Thawne (Barry 1) and saves the kid Barry (Barry Prime). Then in one of the seasons Barry travels back in time and hides in a closet to save his mum (Barry 2) but Barry 1 tells him not to do it. Then at the end of season 2 another Barry goes back in time and saves his mother (Barry 3) and then another Barry goes back in time with the help of Thawne to stop the other Barry from saving his mum. So in total we have four Barry Allens in the same room. How come none of them collided or saw each other (except Barry 1 and 2 as I explained)?
When Barry went back to save his mom, there were 3 Barrys in the room. Since Barry stopped his mother from getting killed, an alternate timeline was created in which Barrys mom was not killed. Now when Barry 4 took reverse flash back to kill his mother, another Timeline is created. Now Barry's mom exists on 3 different timeline and Barry only exists in one