With Shadegg’s unexpected retirement, that brings the total number of open seats that the Republicans will be defending to either 22 (per CQ) or 26 (per Ambinder) or 29 (per Politico), plus four vacancies of members who have already resigned or died.  Many of these open seats are in safe Republican territory, but at least eight are toss-ups and four are only leaning Republican.  The GOP has at least 28 vulnerable (toss-up or lean Republican) seats in this cycle. 

P.S. To put this in context, Democrats lost 22 open seats in 1994.