The protocol includes the primer pair sequences for the V3 and V4 region that create a single amplicon of approximately ~460 bp.
From the Illumina 16S Sample Prep guide, assuming that is the protocol being used.
It's 460 rather than 420 because this library prep includes the primers on the reads. So, there is 420nts of biological sequence, but 460nts of sequenced amplicon. Right now, everything except a crazy short archaea 16S is being lost in merging due to lack of overlap.
You need to increase your trunc-len to keep at least 460+20+BIOLOGICAL_LENGTH_VARIATION total bases. 485 or up would be a good target.