To your first question - using a more broad definition of "retiring", and assuming you're really curious as to the number of Republicans leaving for whatever reason...
According to the "Casualty List", on the US House of Representatives' Press Gallery's website, the list of Republicans "Left or Leaving Office" is 50 people long:
Noem (R), SD (Governor)
Zinke (R), MT (Administration)**
Pompeo (R), KS (CIA)**
Johnson, Sam (R), TX
Price (R), GA (Administration)**
Jenkins, Lynn (R), KS
Mulvaney (R), SC (Administration)**
Renacci (R), OH (Senate)
Chaffetz (R), UT**
Ros-Lehtinen (R). FL
Labrador (R), ID (Governor)
Reichert (R), WA
Dent (R), PA**
Trott (R), MI
Murphy (R), PA**
Blackburn (R), TN (Senate)
Tiberi (R), OH**
Duncan, John Jr. (R), TN
Barletta (R), PA (Senate)
Messer (R), IN (Senate)
Black (R), TN (Governor)
Rokita (R), IN (Senate)
Pearce (R), NM (Governor)
Jenkins, Evan (R), WV (Senate)
Hensarling (R), TX
Smith, Lamar (R), TX
Poe (R), TX
LoBiondo (R), NJ
Goodlatte (R), VA
Barton (R), TX
Franks (R), AZ**
Farenthold (R), TX**
Shuster (R), PA
Harper (R), MS
Royce (R), CA
DeSantis (R), FL (Governor)
Issa (R), CA
McSally (R), AZ (Senate)
Meehan (R), PA**
Frelinghuysen (R), NJ
Gowdy (R), SC
Rooney (R), FL
Cramer (R), ND (Senate)
Costello (R), PA
Ryan (R), WI
Ross (R), FL
Bridenstine (R), OK**
Garrett (R), VA
Pittenger (R), NC (lost primary)
Sanford (R), SC (lost primary)
*died in office **resigned