Variations of this question are one of the most common for people getting started with WordPress, but the answer—as is the case to most good questions—is “it depends.”
A “plugin” is not a “plugin” is not a “plugin.” The plugins recommended on this site vary wildly in size, features, and complexity.
One plugin, WP Comment Humility is a single file and less than 100 lines of code. I’ve used some that are barely 10! On the other hand, Jetpack is dozens of files and thousands of lines of code. You could install tens of small plugins before you even began to approach the complexity of just one large plugin.
So to answer the question as best I can, use as few plugins as possible but use as many as you need. Modern web servers can handle quite a few well-coded plugins, but you never want to push your luck if you don’t have to.
The number of plugins you install does not just affect server performance. Every time you add a plugin, you’re adding a potential source of security problems, bugs, plugin conflicts, and administration complexity. Plugins are an amazing part of WordPress, but, as with many good things, are best in moderation.
Every time you consider installing a plugin, double-check whether you need it at all. And once you’ve had your site for a while, return to it, see if you’re still using every plugin, and uninstall any that you don’t use anymore.