You have two separate interfaces to consider in your question:
1) The ISP connection to your home. This will be depend on just what your ISP actually allows, which is dependent on the package you are signed up for, and any characteristics of the connection cabling that mean the package speed cannot be reached.
2) WiFi. Each WiFi network is a separate shared connection between devices and the origin of the network (normally a router or wireless access point).
Both the ISP connection and the WiFi network can limit speeds. If there are multiple devices using the WiFi network, then these devices are all sharing the same WiFi, and the total speed over the WiFi will be sum of the network speed being used by each device. Similarly, the total speed over the Internet connection will be the sum of the Internet speeds being used by each device.
If the WiFi is significantly faster than the ISP connection, then adding extra devices that are using the Internet will mean that each device will get less Internet bandwidth once the ISP connection reaches its maximum speed. Similarly if the WiFi speed is less than the ISP speed, then the WiFi will limit the total speed.
It should be remembered that there might be local traffic between different local devices that are not accessing the Internet at all, but are using bandwidth on local networks (WiFi or Ethernet).
As an example, I have a dual band WiFi router. When I stream certain TV/Movie sites through my laptop and then to a Chromecast dongle connected to my TV, the video stream comes from the Internet then over WiFi to the laptop. The laptop displays the show on its screen using the Chrome browser, then it re-encodes the video and sends it by WiFi to the Chromecast dongle. That link to the dongle means packets going from the laptop to the router, and then from the router to the dongle.
The video stream effectively passes 3 times over WiFi. Putting all this over the same WiFi network provides a jerky image on the TV when scenes are changing rapidly. However, if I use WiFi on one band for the link to the laptop, and the other WiFi band to connect to the dongle, the images on the TV show no jerkiness. The Internet traffic is the same in both cases, but passing the video 3 times on the same WiFi network becomes a bottleneck.
(Before anyone comments, some websites will pass video traffic straight from the Internet through the router to the dongle without going through the laptop.)
I hope this helps.