Can I put a monitor next to another monitor while both are connected to my MacBook Pro? I have two monitors. Just for my setup, I was wondering if I could position one monitor on the left of my 15" 2018 MacBook Pro, and then another one on the left of that.
You are looking for a setup similar to this:



This is a commonly used multi-monitor setup and pretty simple to configure.

You can either setup the external displays to extend the Desktop or mirror the MacBook display.

Once you have connected the monitors and have set them up physically, arrange them by going to System Preferences → Displays.

Refer to the Apple Support document, Use external monitors with your Mac for help and more details.

Your MacBook can drive upto 4 external displays. Refer to the Apple Technical Specifications document, MacBook Pro (15-inch, 2018) - Technical Specifications for details:


  Video Support
  
  Simultaneously supports full native resolution on the built-in display at millions of colors and:
  
  
  Up to two displays with 5120-by-2880 resolution at 60Hz at over a billion colors
  Up to four displays with 4096-by-2304 resolution at 60Hz at over a billion colors
  
  
  Thunderbolt 3 digital video output
  
  
  Native DisplayPort output over USB‑C
  VGA, HDMI, DVI, and Thunderbolt 2 output supported using adapters (sold separately)
  




Image Source: https://www.imore.com/sites/imore.com/files/styles/xlarge_wm_brw/public/field/image/2014/01/4k_macbook_pro_hero.jpg?itok=vVPFgzZo