Plenty- but they're pretty much pointless as they aren't 'real' followers. A lot of them are fake accounts or bots. Better to get some cheap software like Tweet adder that will add 1000 people a day... you can normally expect around 50-75% of those to follow you back, with the added advantage that they'll be a lot more likely to pay attention to your tweets.
I think the best thing is to look manually for people in the same niche to be honest. That's what I do, and it feels like this is a more effective way than to just add random people.
BTW, what rules should I follow for adding people on twitter? Example, not adding more than 1000 a day.
I haven't seen such rules on twitter.. though on following if you have to give an interval time..do not follow one after another on a high speed..
I suggest you skip all kinds of software and search for people on twitter instead. You know they have a real-time searching tool, right?
that's a great advice, but it's a lot easier to do it with various software (like TweetAdder)... but if you don't want to spend any money, using the twitter search tool would do.
Go to http://search.twitter.com/advanced add your keywords in the words section, and you'll get all the latest tweets that includes those words. Follow the people, based on your search.