Safe versus Competitive Justification
If a U.S. House race was won by more than 10 percentage points in 2016, it is considered safe for 2018.
It is extremely unlikely (but not impossible) that these safe seats will flip from one party to another.
There are currently 213 safe Republican seats
There are currently 180 safe Democrat seats
28 Republican controlled seats may be competitive in 2018
14 Democrat controlled seats may be competitive in 2018