How many plugins is too many?

There is no hard number, but each plugin adds load time and maintenance. Install only what earns its place and keep everything updated.

What is the one plugin a contractor should start with?

Livable Forms. It is free and gets a lead-capture form on your site immediately, which is the first thing that turns visitors into jobs.

A lean, updated stack with a caching plugin stays fast. Bloat comes from installing many overlapping plugins you do not need.

The short list of WordPress plugins that turn a contractor site into a lead machine: page builder, SEO, speed, security, and lead capture.