Currently, there is no way to count services or elements that are in alarm for a given alarm filter using the AlarmSummary shape. You can find all available statistics for this shape in the help.

Feel free to suggest this as an addition to the AlarmSummary functionality on the feature suggestions page.

Edit:
With the above being said, there is a way to visualize the number of services in alarm.

The solution is not straight forward:

1. Configure a children shape that creates a shape for each service in alarm. You can set the scope for this to a certain view. Additionally, you can create a severity filter that limits the services to those which are in alarm (and even configure in which severities you're interested in).
2. Add the 'Highlight' shape data to the service template shape and set its value to 'true'. This will cause the created service shapes to be highlighted at all times.
3. On the Visual Overview, we can have a shape that counts all highlighted shapes. In the shape text, we can then use the [Highlighted] keyword which will be replaced with the count of all highlighted shapes. This will be identical to all services being in alarm.

I have created a Visio file that contains this setup. Unfortunately, we can't upload documents on here yet so feel free to email us with a reference to this question so we can mail you this Visio file in return.