Dynamic template using Iron - URGENT help pls!

I have a requirement which is as follows.

When the page loads show a default template(dame div tags but different data) based on a session parameter which is set based on an api response. e.g if the customer has 1 cctv show only one camera view, if customer has 3 cctvs show 4 thumbnail view and if customer has 6 cctvs, show 9 thumbnail view
The default template can have 3 different layouts and only one layout is selected to show on load of the website .

I am trying to use iron:dynamic templates but unable to get good documentation on how to use it. Can someone pls help?