What is the limit of circuits. For example if I open 100 unique domains will TOR make 100 different circuits or does this have some upper limit?
In Tor Browser, every domain gets it's own circuit. The Design and Implementation of the Tor Browser document has some more details on it.
Frankly, you get an own circuit for every domain shown in the address bar. So, if you visit google.com a different circuit is used than when you visit example.com even when content, like an advertisement, is loaded from google.com.
I do not believe that there is a limit or at least not one you're likely to ever hit.