You can’t give captions in portfolio templates.
Following points will be helpful to you in order to understand the logic,
1. Portfolio Templates like 2 column, 3 column etc are placeholders for your portfolio items. e.g. I have portfolio say “Contango WordPress Theme”, so main screenshot of my Contango portfolio will appear on the placeholder that will link to the detail of Contango portfolio post.
2. As i explained in the point 1, Placeholders link to the main portfolio post. Now, the Portfolio Post is just like a Blog Post. You can add text, images, quotes etc to describe your portfolio in detail.
As it just like Blog Post, you can use standard tags of WordPress. In order to create multiple pages you can use