It's important to understand how big your targeted audience is before you create any activity, whether it's an email sending or an offer on your website. By creating a Segment in APSIS One, you can see how many Profiles are included in the segment for you to fully understand the potential in your campaign.
As you create your Segment, you will be able to see how many Profiles match the Segment you're creating:
Click Calculate Profiles on the Bottom bar.
And the number will be updated. You will have to refresh the Profile count every time you make changes in the canvas.
This feature is also available in other tools, like the Email tool for example.
In the Send to step, you're able to Calculate Profiles after you've selected the Segments to include or exclude.