The only answer to that question is probably “As much as there is”. Plugins add better funtionality to your WO blogs and allows you to so much more than the typical Wordpress install does.
Sometimes however, you may be shocked by the amount of plugins a person has. This site’s post is about a blog that has 35 plugins:
Smuggle Me » 35 Wordpress Plugins on One Blog
You may want to check what plugins are used and get them for yourself. I’m beginning to think 35 is still not too many 
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