An extent is a contiguous 8 pages. So, given that the first page in the database is page 0, the first extent is made up of pages 0..7. The second of 8..15, etc.

Hence, while extents aren't actually numbered within a database (pages are), one could say that an arbitrary extent 'n' consists of the 8 pages from (n-1)*8..(n*8)-1.

Make sense?

I'm curious though. Why are you interested?