You can use as many Webfonts as you like, as long as each one is defined in your style sheet. But if for example you have an iframe in your page which contains a webfont that is not defined on the stylesheet of your page, then there can be extremely strange loading effects in IE, esspecially IE7 & IE8.
The following images show the loading process of the Meyerstollen page before the problem was fixed. State 1 and 2 are using completely the wrong fonts. It took about 10 seconds to get to the right state.
State 1
State 2