Reading a Property from UI into the Model

[ProphetLamb]

My app has a setup wizard built using a TabView and a set of TabViewItems. Each TabViewItem is a bespoke xaml control, they share a Save & Next button.

I wish to implement the following behaviour:

Given not the last TabViewItem is visible.
When Save & Next is activated.
Then the next TabViewItem is visible.

Given the last TabViewItem is visible.
When Save & Next is activated.
Then the Page navigates back.

Currently I hardcode the number of TabViewItems in the TabView in.

I determine whether the current tab is the last one, by binding the SelectedIndex of the TabView, and comparing its value with the total number of tabs.

public int WizardTabsCount => 6;
public IState<int> WizardCurrentTabIndex => State.Value(this, () => 0); 

How would I bind the property WizardTabsCount to the value of TabView.ItemsCollection.Count.
I absolutely wish to avoid a reference to the ItemCollection in my model, executing code outside of the UI thread.

Implementing a converter from ItemCollection to int and then twoway bind the ItemCollection to an empty state WizardTabsCount, yields a binding error.
How should I best go about this?

This implementation is preparation for hiding certain steps, removing TabViewItems dynamically, if the user needs not fill them.