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″
I have no affiliation to EveryMac, their partners, advertisers or sponsors.