The exchange for each contest is different.
Go to www.contesting.com
and down near the bottom of the page, there is a contest calendar.
The contest calendar has links to the specific rules for each contest.
Click the individual link to find out the exchange for the particular contest.
MOST contest numbers are consecutive serial numbers.
Your first contact in the contest is #1, the second is #2....etc.
In some cases, the numbers exchanged are member numbers, such as 10-10 club member numbers.