Depends on what the plugins do
If your talking 20 plugins to provide admin panel features, yeah - that might be a bit excessive.
If its a few for front end stuff, a few for admin panel stuff, a few for security, a few for SEO - I don't think it's an issue
I've built Wordpress websites that didn't use any plugins and had custom code to handle anything additional and I've built sites that has 10+ plugins for various purposes. Both performed the same. Plugins generally target a specific feature or need: SEO or adding additional fields to the content editor or security or etc... So so long as there aren't 10 plugins, all dealing with SEO - should be ok.
Also - take into account the server. A small instance (1gb ram) at a cloud host with 20 plugins may perform much different then a website on a 4gb instance with 20 plugins.