App question how many ipads

I have a question- I own an ipad and may want to purchase a second one for the family. The question I have is I have already purchased an app for my first ipad can I also use it on the second? Or do you have to re-buy the app?

Thanks in advance

As long as you sync it to the same library (which obviously means on the same computer) as the other iPad, you don't have to buy it twice.