It's not simple at all. Where does the harvest stop?
If we accept the suggestion made in this thread that the Reapers' mandate extends to all organic life, everywhere, and not just in the MW, they must keep spreading to every new galaxy that they can. Chaos exists in every galaxy, remember?
You can't just put in an arbitrary boundary like "oh well they only go to the ones in our local cluster", since people are already rejecting the much more sensible idea that the Reapers don't venture beyond the Milky Way. Why would there be an arbitrary limit like that, if you're insisting on the mandate applying universally?
Eventually the cumulative travel time between all those galaxies is going to be larger than 50,000 years, and they are going to need more fleets.
It is far more likely that what was presented in the games was true, and the Reapers only harvest the Milky Way. They hibernate for 50,000 years between cycles, and they've never been to another galaxy because their programming only makes them interested in this one.
There's also the fact that Leviathan makes continual references to the galaxy during your chat with it - up to and including stating that "The galaxy itself became an experiment." It did not speak of "The Local Cluster", "The Universe", or anything outside of the galaxy in any context. If the reapers were ever involved in anything outside TMW, it was without the knowledge of the Leviathan.