Oh my, now you are going to get very confused. No, the number does NOT go down in sequence for the studies. The numbers are just publication of collections of his works (in this case, his studies). For example in Op. 823, there are 73 exercises!
Czerny was a prolific composer and he wrote tons of studies for the piano. There are near countless collections of studies by him: Op. 299, 261, 599, 818, 819, 829, 821, etc, etc, etc, etc..... and the famous 822. Try checking out:
https://en.wikipedia.org/wiki/List_of_compositions_by_Carl_Czerny. To navigate through the many opuses of studies, you probably need a teacher to guide you. But as I suggested, start with 821 and then just jump to 299. It should keep you busy for the few years. Besides, who knows if you want to try studies by Burgmuller or Heller?? Heller is another one that wrote a whole bunch of studies as well, for eg. Op. 45, 46, 47, etc, etc, etc. But again, you are only in Grade 3, so either Op. 823 should work, or Burgmuller Op. 100.
Now I feel guilty for not being of much help, but merely confusing you. As I have qualified myself time and again, it is best, perhaps, to seek guidance from your teacher?

Let me know what you think.