As a developer, I find the great bang for buck is two monitors, with minor benefits for each additional monitor. Examples of use for extra monitors:

• code on main monitor (or more if I have multiple projects open)
• app I'm working on
• documentation
• Stack Overflow/Search/AI chat
• Team chat/Slack
• Email
• infrastructure/performance monitors, logs
• Mandatory reboot notice from the IT department shoved in a corner until I can reboot without losing work

Of course if you have a large monitor you can tile windows instead of having N screens. Virtual desktop also exist.