When I buy a song on iTunes does it stay on my account?

The problem is that I have an iPhone and I want to buy a song on iTunes but soon I'll get a new iPhone, if I enter my account in my new iPhone (with the song already bought in my old iPhone) does the song appears in my new iPhone?
If it doesn't what Do I have to Do?

Yes it does but I think there's a limit to
How many times/devices you can download it

It will always be on itunes. You can look on the right side of the itunes store where it says "Quick links" and click purchased to get anything you've purchased in the past, no matter what iphone you use to connect.