100 + Number Items, maybe. But the Rules can be made generic. You should have only one set of Rules. Never ever copy, paste, edit to change the Item names in order to add support for a new Item. The Rules can be made generic so all you have to do is add the new Item to a Group.
Yes, you can create a Map using the Item name as the key and anything you want in the value.
Yes, Groups would be the best approach. But you will still have to put the result into a new Number Item. Also note that you may need to deal with threading which might make this a bit complicated. It depends how likely the changes are to occur really close together to the Group. Look at Design Pattern: Associated Items for how you can make a single Rule to handle all of these calculations.
The example you linked to was written before the introduction of the Member of Rule trigger.
You can’t display a chart like that in BasicUI. Your best bet is to save all these values to a database using Persistence and generate a chart like that using Grafana (it does way more than just line charts).
In HABPanel, a custom widget would need to be created.