It doesn't matter how many there are. It's a huge, huge area of space. In 39 years, Voyager 1 has traveled 119 AU. That is 0.0018 light-years. In order to reach Alpha Centauri (it's not headed in that direction), the next-closest star at 4.36 light-years, it would have to travel 2,320 times the distance it has already traveled. Or, put another way, if the distance to the next star over were the length of a football field, Voyager 1 - the furthest man-made object from the sun - has traveled a whopping 1.4 inches.Andromeda is 2,530,000 LY away from us. 580,275 times farther away than Alpha Centauri. Try to wrap your head around how much empty space there is between us and Andromeda. It's not like finding a needle in a haystack. It's like finding a single water molecule in the ocean. The Reapers have been around for two billion years. They've had one cycle every 50,000 years, so there should be at least 40,000 Reapers in existence. Even if these Reapers were able to see everything that goes on in a 1-LY-radius, and they spaced themselves into a circular two-dimensional search grid 1-LY apart, in 360 1-degree "arms" 111 Reapers long, and then faced this grid towards the Milky Way, they would still only be able to cover an area of 222 LY. If you put a 222-LY-wide circle halfway between us and Andromeda, it would look like a tiny little speck.Again, mistakenly encountering Reapers on the way to Andromeda would be like hitting a human hair clutched in a pair of tweezers with a sniper rifle from 2000 yards away. They wouldfind the Arks during their voyage. Ever. It would be far more likely that they would indoctrinate someone left behind in the Milky Way who knew about the program, and then uncover precise details of the mission that way. And then, of course, their entire mission is restricted to harvesting life in the Milky Way, anyway, so even if they knew, it's unlikely they'd dispatch Reapers to go investigate.If you want an idea of the scale of the Universe and how incredibly small we really are, here, try this: