I just finished reading Plutarch's biography of Ceasar and am puzzled as to how exactly his famous march on Rome worked. According to Plutarch, Ceasar marched on Rome with only 5,000 infantry and Pompey, upon hearing this, declared the city to be in a state of anarchy and fled the capital. I know that Plutarch isnt the best source (if only we had livy!) and that Pompey was being pulled from multiple sides and was unable to make a clear decision, but still, the facts puzzle me. Did Rome not have a wall around it at this time? Was there really no garrison or peace keeping force or hell, enough people with weapons to man the walls? Any additional light anyone could shed on this critical moment in Roman history would be greatly appreciated by me.
TL:DR Pompey and the Senate probably believed that Caesar had more than one legion with him.
As you point out, attacking Rome with only one legion probably would not have gone very well for Caesar: there were walls, there was a garrison (albeit a small one, Rome at this point relied mostly on its laws and sacrosanctity to keep it safe, which was why the Praetorian Guard of Augustus was such a big deal), armed and ornery citizens, not to mention Pompey Magnus himself, war hero and living legend. It's possible that Caesar never intended to take the city at all, but rather leverage his troops being outside the walls into a senatorial appointment (a la Octavian), amnesty from prosecution or even just a meeting with Pompey.
Anyway, attacking Rome with one legion was crazy, which is a big part of why the Senate assumed Caesar probably wasn't doing that. No one would be willing to break custom, law and the will of the gods by crossing the river, and he certainly wouldn't do so with only one legion. Of course, never mind what it means about Caesar, it says a lot about his troops. These were Roman soldiers the likes of which hadn't been seen since the end of the first Punic War: battle hardened, able to move fast for hours and days on end, and fanatically loyal to their general. And Caesar had several legions (just not with him at the time).
Once they'd crossed the Rubicon, Caesar did a lot to add to the confusion and fear in the city. First and most famously, he was fast. When the Senate thought they had time to deal with the problem, reports kept coming in that Caesar was one or two steps closer than they could believe.
Second most famously, he let people go. Even if they thought he was a criminal, even if they planned on opposing him later on. This meant that some people fled during an attack, others probably headed towards the city if not calmly, at least not at full-out speeds. So the Senate was getting reports (individuals will always move faster than even one Legion) from dozens of people at the same time, some from cities miles and miles apart. This would certainly look like more than one Legion.
And it's important to remember the point you made, about Pompey (who was "sole counsel" at the time, not exactly a straightforward political or military role intrinsically) being pulled in a lot of different directions, some Senators saying that abandoning the city was folly, others already packing their bags, others insisting we discuss this one more day or take some more omens, bickering and debating and countermanding and blah blah blah. "I am Pompey the Great! Get me away from these bloody Senators! Get me some troops!" Specifically, get him some of those troops he used when he conquered further East than anyone in the history of the world (other than Alexander, but, come on, it's Alexander, no one is gonna match Alexander (certainly not that little blonde whelp nephew of Caesar's))
Three months late, I'm sure you've already looked all this stuff up, but it's Finals season and I don't want to study :)