There are a lot of pitchers who are described as "RP" but who started most, if not all, of the games played in individual years. 1950 Jim Hearn is a good example. He comes up on rp searches but started all 16 games he played in 1950 and pitched almost 8 innings per appearance.
There are literally dozens of similar pitchers who aren't really relief pitchers but are described as such by WIS.
So, are there any limits other than being called a rp?