All very valid questions!

Some companies are going to come into Twitter and gain huge followings instantly because they already have name-recognition and brand loyalty. See: Starbucks

In terms of keeping track of everything, this is where you need a really good Twitter app comes in handy, like TweetDeck, HootSuite, etc. Depending on the app, you can create follow lists, sort users, create autosearch features that will comb the massive amounts of info out there for you. Never do anything manually if you can automate.

In terms of listening, there are some real tangible benefits to listening--and responding. I had some issues with Turbo Tax two weeks ago, tweeted about it, and in less than 10 minutes two TurboTax experts DMed me to ask if I needed help. That's huge. It saved me a lot of trouble, and it gained them a lot of credibility.

If you don't have a ton of time to devote to a good Twitter feed, a smaller following will probably be easier to deal with. In my opinion, it's always quality over quantity. There are tools out there that will gauge the level of interaction with your followers. If your followers just follow, never DM or RT you, that's not a quality follower. You want someone who will interact (it is social media, after all), not someone who's a blind follower.

However, some people judge you by your raw numbers only. Don't be fooled. Of the zillion people who "follow" Aston Kutcher, how many are doing it just because their friends are?

Good luck!