If i say 1 in 300 people will have this,what does it mean

what does 1 in 300 mean

1 out of every 300 people
or approximately 0.3% of all people.

http://www.algebrahouse.com

it means that in a group of 300 people it is likely that one would have "that"
or it also means that there is a 0.3% chance of someone having "that"

It means out of every three hundred people one person would get it and it is not very likely