This is a difficult question to answer. Almost all WordPress blogs will have at least a few plugins, but the number of plugins your blog can support may be determined by several factors. One thing is clear: too many plugins can slow down your site, so you should choose them wisely.
WordPress is a dynamic blog content management system that uses a combination of PHP (an open scripting language) and MySQL (a free and open source relational database management system) to create web pages on the fly, as they are requested. This means that every time a user clicks a link that takes them to a page on the blog, the web server has to call up the database and create that page.
Every WordPress plugin adds additional PHP code to WordPress and must be loaded whenever a user accesses the site. Too many of these little code snippets can add up to a big chunk of extra code required just to get your site loaded and viewable in a browser. While caching and other performance tools can help reduce the amount of time it takes to load pages, there is no substitute for using discretion when installing plugins.
Another reason to be careful about the number of plugins you install is that you have to maintain plugins just as you maintain your WordPress installation. They need updates, and some lower-quality plugins may stop working with newer versions of WordPress, while the developers are long gone and have no intentions of providing updates.
The key to good plugin management is to start with the plugins you actually need. There may be a handful, perhaps 3 to 5 plugins, that you must have on your blog. Others may seem attractive, but only add those that will actually be of benefit. If you stop using one, do not forget to uninstall it.