There isn't a strict limit on the number of dashboards.
However, the response times of navigating through your QlikView app will depend, internally, on it's design and the way the app has been optimised and ,externally, on the processing capability of your server and your network.
If you need help with optimisation and design have a look at the blog link below:
Try not to throw everything into one app. Often an app that tries to meet every user's requirements tends to be compromised and leave all users dissatisfied.