Again, there will be 8 dungeons in Dragonflight.
4 while you’re levelling and an extra 4 when you hit end-game. In M+ Blizzard is doing in Dragonflight what they’re doing in Shadowlands S4. Which is that 8 dungeons will be available for the first season of M+, 4 from Dragonflight and 4 from previous expansions. Every season these will rotate, so in S2 there will still be 8 dungeons available for M+ but it will be a different 4 from Dragonflight and a different 4 from previous expansions.
By rotating in dungeons from previous expansions they can add M+ modes to some of those over time (if they didn’t have any) while also keeping things fresh (in a sense) so players aren’t doing the same 8 dungeons over and over and over again season after season.