Apple - MacBook [Pro] - how many displays, what resolution/frequency?

^{This may not be the only available resource on this topic, but it is a comprehensive & reliable one...}

EveryMac, as the name suggests, has information on every Mac model ever made [also iPhones, iPads, iPods], including but not limited to the number of external displays supported & their maximum specification.

If you are not absolutely certain which model you have from  Menu > About this Mac, then they have a search engine which can identify from many data types, serial number, part number etc, the

Ultimate Mac Lookup

Lookup Macs, iPods, iPhones and iPads by Apple Order Number, Model Number, EMC Number, Model Identifier, Serial Number and Intel processor number.

This is an example of the data they provide for the current 2018 Core i9 15"

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@Tetsujin answer includes very good third party options. But I often find direct information is better instead of going to a third party.

So get information directly from Apple...

1. Navigate to Apple's specs website

2. Get the Model Identifier from Apple Logo > About This Mac > System Report > Hardware > Model Identifier

3. Copy and paste that identifier into the search bar on the page form Step #1

4. Click your product (should be the first one)

5. Relevant Information can be be found in the following sections:

   • Display
   • Graphics
   • Video Support
   • Electrical and Operating Requirements

For example here is the "Video Support" sections for MacBookPro14,2:

---

Video Support

Simultaneously supports full native resolution on the built-in display at millions of colors and:

• One display with 5120-by-2880 resolution at 60Hz at over a billion colors

• Up to two displays with 4096-by-2304 resolution at 60Hz at millions of colors

• Up to two displays with 3840-by-2160 resolution at 60Hz at over a billion colors

  Thunderbolt 3 digital video output

• Native DisplayPort output over USB‑C

• VGA, HDMI, and Thunderbolt 2 output supported using adapters (sold separately)