You can use a data query to get at raw data like the activity log counts by worker over the past week. This can help you estimate seat usage and if you have the correct ammount.

You can add these queries to a chart or run then in the data query tester found in Setup » Developers » Data Query Tester:

Use the following query:

type:worklist.subtotals
of:activity_log
by:[actor~500]
query:(actor:worker created:"today -1 week")
format:pie

You can filter workers by group like this:

type:worklist.subtotals
of:activity_log
by:[actor~500]
query:(actor:worker created:"today -1 week" actor.worker:(group:(name:"Support")))
format:pie

You can also show the number of distinct workers each day in the activity logs:

type:worklist.subtotals
of:activity_log
by.distinct:[created@day,actor~500]
query:(actor:worker created:"today -1 week" actor.worker:(group:(name:"Support")))
format:timeseries

That won't tell you how long each session was, but it gives a general idea of activity per worker.