How many advisors can a team leader effectively manage

Good question! I have managed up to 38 people and as little as 10 before. Each has its own advantages and disadvantages.

I suppose that it first depends on what type of call center you have, then on the quality of the advisors that you have (and ultimately how much support and 1 to 1 time they need).
Then it depends on what targets do you as a team leader have for your agents? Are you tasked with doing personal development with them, what stats do you feed back to them, how much is this data automated, 121's etc?

If you have 10 staff and have to get call stats (ringing time, lost calls, average call duration etc) and everything is manual it will be very time consuming. If its fully automated on a daily basis you have this information at your fingertips so delivering it to 30 people would only take slightly longer than 10 people - plus daily stats let you trend each agent in 'real - time'.

Not really an answer, more like more questions that you need to answer yourself before you decide if you need another team leader.