The fastx_split command divides a FASTA or FASTQ file into roughly equal-sized
pieces. The size of a piece is measured by the file size in bytes rather than
the number of sequences or number of sequence letters.
-splits n option specifies the desired number of pieces. You may get n+1 pieces.
The -outname option specifies a pattern for the output
file names, which are always FASTA. Ther must be a '@' character in the pattern,
which is replaced by the split number 1, 2, 3...
usearch -fastx_split db.fastq -splits 10 -outname