>>14490392
Correctness has 8, (but one of the Correctness chapters is actually an additional Transmitter chapter with Sumio) Matchmaker has 5 and Placebo has 7. Overall I'd say it's a few hours longer than Silver Case. It's also recommended (but not mandatory) to play each part step by step. So, Correctness -> Matchmaker -> Placebo 01, and then 02, etc. The 00 chapters for Correctness and Placebo are the tutorial and recap of Silver but still should be read, of course.
>>14490408
The only new chapters are the last ones in both games that link to the next game. 25th hints at a brand new Silver Case title it seems. Kamuidrome in Silver was definitely part of the original, and almost all of Placebo in 25th is basically a different Lain. I don't think that's newly written. However, I do think some (((localisation))) may have happened.