How many WordPress plugins are too many?

There is no fixed number. A site running thirty well-written, lightweight plugins can outperform and outlast a site running five badly written ones. The number itself is not what causes problems — what each plugin does on every page load, and how well it is maintained, is what actually matters.

Why the number is the wrong question

A plugin that runs one simple check and does nothing else costs almost nothing, no matter how many others are active alongside it. A plugin that runs dozens of database queries on every single page, loads its own CSS and JavaScript everywhere regardless of whether it is needed, or makes an external request to a third-party service on every load, can slow a site down on its own — and would do exactly the same amount of damage whether it was the only plugin installed or the fortieth.

Counting plugins tells you nothing about which of these two situations you are in. Measuring what actually happens on a page load does — see finding the plugin that is slowing your site for how to do that properly rather than guessing from the count on the Plugins page.

What genuinely does scale with plugin count

A few things do get worse the more plugins are active, independent of how well any individual one is written:

• Security surface. Every active plugin is more code that could contain a vulnerability, and more code that needs its own updates tracked. This is the strongest argument for keeping the number down, since it scales with count regardless of quality.
• Update burden. More plugins means more updates to review and more chances that one of them conflicts with another after an update.
• Compatibility risk. Two plugins rarely conflict directly, but the more that are active, the higher the odds that some combination eventually does — particularly plugins that both try to modify the same area, such as two SEO plugins or two caching plugins running together.

A more useful way to think about it

Rather than aiming for a target number, periodically ask of each active plugin: is this still doing something the site needs, is it still maintained, and does a lighter alternative exist that does the same job? A site reviewed this way every so often naturally settles at whatever number of plugins it actually needs — sometimes five, sometimes forty — without carrying dead weight either way. See is WordPress secure? for how plugin choice and maintenance connect to the wider security picture, not just performance.