I am glad it is finally here. The Very 1st batch of Su-27SK PRC bought from Russia was 26 in total (20 SK & 6 UBK). So this 24 may very well be an initial purchase.
I strongly believe a lot depends on the progress made by SAC. If the J-11D turns out to be a supercruising flanker then they may not need more Su-35SK. Otherwise, I think there will be more.
I 've said this before, I'll say it again, PLAAF alone has something like 200 old flankers with no real upgrade path as far as we know (Close to 300 in total if you include PLAN). Indian Air-force alone has over 200 Su-30MKI sporting NIIP N011M, ELTA El/M-8222, and more . With Su-35SK, the PLAAF has a flanker superior to the MKI in every way. These Su-35SK should also hold its own against USN F/A-18E/F.
Yes, the J-20 is the best the PLAAF has, but it is going to take time for the PLAAF to adapt to its capabilities. It took along time for the USAF develop new tactics for the F-22.