The Send Limit feature on the Review page allows you to set the total number of recipients for your campaign. For example, if your estimated number of recipients of your segment along with exclusions is 5,000 but you only want 2,000 people to receive your campaign, you can enter this total into the box shown in the screenshot below.
When this campaign deploys, it will only go out to 2,000 random recipients out of the original 5,000 targets.
If you would like to send to the remaining 3,000 recipients, you would just need to duplicate this campaign and make sure to exclude the original 2,000 recipients of the first campaign in the segment tab of your campaign flow. This prevents any users from receiving the same exact campaign multiple times.
When you get to the review page of your second campaign, you can leave the send limit box blank if you’d like the campaign to go to all of your remaining users in your segment or you can change the send limit amount if you want a specific number of recipients for this campaign.