[quote=@Silver Fox]
<Snipped quote by MrDidact>

Aye aye will fix it. I know that in lore there was a good portion of Geth that didn't get involved. As it was discussed that the Reapers offered but many didn't. As I remember at least. But that may have been in ME2. But I'll fix it anyway
[/quote]

Prior to ME3, the geth were stuck in a split of whether to follow Sovereign or not. 

During the events leading up to ME3 and ME3 itself, the quarians attacked and were more or less unanimous in accepting the Reapers as a means of saving themselves from the quarians. You're literally seeing the entirety of both species locked in a total war.

From what I recall, Legion was unique among geth because all of its runtimes were local in that body. Usually geth all "live" in massive communal servers and the platforms that are their physical bodies are more or less puppets that they don't really lose much if it is destroyed since they are remotely connected. It's like an avatar, as it were.

Of course, the GMs can easily hand wave that the geth had more scouting units like Legion and say Legion itself wasn't aware of any others.