Since these are paired-end reads, N input == N forward == N reverse, not N forward + N reverse. But yes you have the right idea.
These are the # of reads left after each step of the process: filtering based on Q score, denoising sequences, after merging sequences (the drop here indicates paired-end sequences that cannot merge, either because the sequences are too short and do not overlap, or because the ends do not align), and after chimera filtering.
Not all samples have the same value, but most do. Evidently your sequences contain very low levels of chimera. That’s a good thing!