First, you need to know which Council Tax band the property is in. If you don't have that information, you can find it here:
http://www.voa.gov.uk/council_tax/cti_home.htm
Then you need to consult the table here:
http://www.westminster.gov.uk/councilgovernmen tanddemocracy/councils/counciltaxandfinance/ba nds.cfm
As you'll see, Council Tax in Westminster ranges between �439.33 per year for Band A properties and �1318.00 per year for Band H properties.
Council Tax does not depend upon the number of residents in a property except that when only one adult lives in the property, the Council Tax payment is reduced by 25%.
Chris