There are 58 Republicans in seats with a partisan lean of +12 points Republican or less. This includes representatives like House Speaker Paul Ryan and Dave Brat (who took out Eric Cantor in 2014). There are an astronomical 103 seats that have a partisan lean of +18 Republican or less. This expanded list includes the highest-ranking woman in the House, Cathy McMorris Rodgers, and the longest-serving House member, Don Young.
Now, I’m not saying all or any of these particular Republicans will lose. Most incumbents win — even in wave elections. Indeed, incumbents tend to do better than the partisan lean of the district would suggest, though that effect has been getting smaller and is usually lessened in waves.
What I am saying, though, is that when the generic ballot is showing this large of a lead for one party, the playing field of competitive races also tends to be correspondingly huge.