There are a few ways you can customize content based on user activity level.
The user_engagement() function will return a score from 1 to 5 based on your user's engagement level:
{switch user_engagement()}
{case 4,5}
Actives ad here
{/case}
{case 3}
Passives ad here
{/case}
{case 1,2}
Inactives ad here
{/case}
{/switch}
Mailing lists will send to your most active users (measured by opens and clicks) first. Therefore, you can use the emailnum variable, which will ascend in send order.
{if emailnum <= 5000}
Special offer here
{/if}