If your existing fish are healthy and the new additions don’t cause stress for them then the chances of them becoming sick with ich are next to none.It's no fun to get a new fish, and ad it to your tank of beautiful fish, and then BAM....New fish has ich! Then they all die!! And you have to start all over, but before you do that, you to wait to let all those nasty little boogers in your tank (parasites) die! It's heartbreaking, not to mention, very expensive!
I have yet to see a single occasion where healthy fish with a well-functioning immune system in pristine water conditions suffered from ich.
The large majority of tanks have ich present along with other parasites, bacteria and viruses.
Outbreaks happen when fish get stressed and their immune system weakens.
Drop in water quality
Bad quality food
In fact you could say the only reason is stress but the root causes of stress are of course what needs to be addressed.