I have a list of items and they are grouped via a certain classification depending on a column.
How can I get that particular number of total items so I can use it on a status list/kpi?
If I understood your question correctly, this article would answer your question.
Basically you enable the group footers in order to display the "Count" of each group. Then you can modify XSL to fine tune the "Count" to how ever you wish it to be.
A Status list (or KPI list) cannot count the items of one "group". However, it can count the items that are shown in a certain view. I sugest you create a new view for every value of the column that you group by.
Example: You group your task list by priority. Your priority column has the values "low", "normal", "high" and "critial". So you create the four views ("low", "normal", "high" and "critial"). After that you create your KPIs and use the new views to count the tasks with that priority