Question

What is the limit on QueryString / GET / URL parameters

Answer 1

There is no limit in theory. For HTTP URLs, the HTTP 1.1 specification states:

The HTTP protocol does not place any a priori limit on the length of
a URI. Servers MUST be able to handle the URI of any resource they
serve, and SHOULD be able to handle URIs of unbounded length if they
provide GET-based forms that could generate such URIs. A server
SHOULD return 414 (Request-URI Too Long) status if a URI is longer
than the server can handle (see section 10.4.15).

But in practice, many clients and servers do only support URLs up to a certain length. The rule of thumb is not to use URLs longer than 2000 characters (percent encoding already taken into account).