I was told no more than seven, followed this and had a solid week of problems and, after following numerous recommendations, found out I had three bad plug ins!

Part of the issue with plugins is they can become memory intensive. This is why it is recommended to use "with care". (i.e. 5)
The more plugins, the more resource hungry your app becomes.

Consider adding something like W3 Total Cache or some other cache plugin. Yea, sounds funny to add another plugin to "speedup" the rest of the plugins but this is a caching plugin that sets up "copies" of your pages that can server instead of recreating the page every time. Only the dynamic content will be recreated. Speeds up your site a lot and saves on server resources. The one got-ya is you have to flush the cache in times of problems and updates.

recommended limit in five, but I recommend not more than 10.