How many people  are using SharePoint in the company?

This can be simply done by querying the database. Try this

SELECT COUNT(DISTINCT tp_Login) AS Users

FROM UserInfo

Note: Not a good practice to query db.

You can also this information in three ways, by default SharePoint tracks 1
months of usage analysis. _layouts/1033/Usage.aspx. You can also use
FrontPage 2003 to get some of this information. Additional history would
have to be accessed programmatically by parsing the log files. We use
WebTrends as a 3rd  party tool to get some of the metrics.

How many sites  are active?

I guess for portal  areas (sub-webs) you can look at the Publishing Dates, if
there is no expiration set up.