The -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...
Example
usearch -fastx_split db.fastq -splits 10 -outname
split@.fa