Can you buy one app for 2 separate IMac's

I have 2 IMac's and am wondering if I buy a Mac app such as Pages can I pay for it once and put it on both computers?

I bought two 2009 iMac's and they have the Yosemite 10.10.4 on them.

I have to install pages, numbers and keynote again as the update is only allowed under the former owners sign on. It's not a free upgrade anymore since I am a new owner....as the apple store people have mentioned something about a 2009 model.