In other words, if there are many Tor users in my part of the world, do I get better or worse anonymity out of Tor compared to someone using it in an area with few Tor users?
If you're connecting directly, then your network observer(s) can see that you're connecting to Tor network. And if you're the only Tor user in that network, the observer(s) can point you out. Although, --assuming you're using the most recent version Tor Browser Bundle-- they still won't have a clue about your activities inside Tor network (e.g. browsing a website).
Thus, the more people use Tor, the harder it is for them to tell who's who.
To hide the fact that you're a Tor user, You should either use (unpublished) bridges or Pluggable Transports.
My guess is that if you're somewhere where Mallory is targeting Tor users, things like deep packet inspect (DPI) will identify Tor users. The fewer Tor users there are, the more they stick out. One reason why everyone should use Tor to diversify the traffic and provide cover.
You question may sounds like: "Does my bitcoin hashrate depends on the number of Bitcoin users around me?"
Onion routing translate your requests through random points over the whole of the Earth.
You can observe Tor map here.
But you may ask: "Does attention to me from special services depend on the number of Tor users around me?"
There are two unpretty answers:
You are in the Niger, there is nobody know what exactly Internet is. Probably. kind of Niger's signal intelligence would mark you as a terrorist number one.
You are in the Europe, amount of Tor users highly great. There are at minimum about 4 legendary signal intelligence agencies united by Interpol. You are on the West side of the World. NSA, FBI, CIA too for you. Looks like again you being marked as someone.