How can I find as much as possible keywords for my search ads campaigns?

Hi,

Just wondering how can I find a long list of keywords for my app campaigns? The method I’m using now is basic keywords + brand name + some identifier words. However, I only have some poor amount, around 60, which just get several impressions every day, far from enough.

How many keywords do you have normally in a campaign? And anyone would like to share your methods to extend the keywords list?

Thanks a lot.