How many screens and data items can I configure on my MI HMI?

On the lowend MI HMIs the runtime database is limited at 92K, so you can have as many screens and points that will fit into that space. You are limited to 100 internal data points however, and only up to 26 on any screen at a time. Only 26 points can have background actions.
On the midrange MI HMIs the database can be up to 384K. You can have 571 internal data items, with 100 items allowable on a screen. Up to 600 data items can have background actions.
I all cases Configuration Manager will tell you how much memory you are using when you 'prepare the project' for download.
The lowend units can easily handle 25 screens, the midrange say 100. It depends on how much data is configured on each.