If Jude Law is playing the version of Albus Dumbledore that is a contemporary of Eddie Redmayne’s Newt Scamander, then we’ll have one flashback that will show teenage versions of Dumbledore and Johnny Depp’s Gellert Grindelwald.
Since Dumbledore is actually quite a bit older than Newt, we’d guess Jude Law will still play Dumbledore during the teenage Newt scenes.
If we’re also going to see a teenage Newt Scamander, one would assume that will be a separate scene, since Dumbledore is significantly older than Newt.
Based on the announcement, it would appear that Fantastic Beasts and Where to Find Them 2 will have at least two significant flashback sequences.