Chris mentioned Jessica Hische’s article “Talking Type” which is a fantastic read but I also recommend reading this section about Webtype’s Reading Edge Series.
+ A good question to ask yourself is, are they truly optimized for the web?
A: Research manual hinting (aka delta hinting) vs auto hinting. This is why majority of Google’s webfonts lack quality. I wish there were more articles on this from Webtype because they seem to do the best job.
A: Typekit opts for JS and states why CSS alone is bad, although I’ve never had trouble with some of their arguments and prefer a CSS solution. Article.
+ Does it matter how many I use?
A: Absolutely because this puts a damper on page loading and requests.
+ Should I test them?
A: Without a doubt. I notice most people choose webfonts based off of the distributors image previews. Those image previews are meant to look beautiful. So just like you would test a website on multiple platforms and browsers, you should do the same for webfonts. It will ultimately save you time.
Some distributors offer a free 30-day period to allow for testing. If you need more time or want to speed up your test phases, I would look into purchasing a plan from Typecast.
+ Opentype features in CSS3 `font-feature-settings` (kerning, stylesets, common ligatures, discretionary ligatures, true small caps, old-style numerals, lining numerals, swashes and many more). There are numerous articles on this.
Note: My knowledge in this area isn’t that great as I’m still learning more about them myself.