now online is a snapshot of the last five minutes. But yes, right now that number is correct in that every single IP loading the site once counts as one "guest" for the next five minutes.
Top 200 albums requires at least 200 votes, with only studio albums featured. Top 20 albums of the year, Top albums by styles, Bottom albums, Live albums and DVDs require 20 votes.
You can create advanced top-lists in the Album index, customizing genres, media types, minimum number of votes, years range and so on.