youre referencing party flips in *general* elections.

the elections taking place right now are *intra-party* primaries. where incumbency and name recognition are even stronger advantages than in generals (party voters tend not to vote against the people they voted for last time).

which is why its shocking/huge news when an incumbent loses in a primary. like when eric cantor lost to dave brat in 2014. or when joe crowley lost to aoc in 2018.

here is a tweet from 2018 listing the amount of house incumbents who lost their primary races each election cycle for the past decade.
https://twitter.com/billscher/status/1037152313406889984

the avg is about 4.3 combined total between the parties. if you know the specific races where incumbents got beat (like the ones i listed above)...its roughly a 50/50 breakdown per party. so 2.15 incumbents unseated per party per primary cycle.

hope that helps.