If you haven’t used it before, or perhaps it has been so long that it has been swapped out to disk, let me commend GNU’s Parallel to you.
Parallel executes shell commands in parallel! A trivial example would be parallel echo ::: A B C, which runs echo A, echo B and echo C. {} can be used as a placeholder for the parameter in cases where it isn’t simply appended to the command line.
Multiple parameters can be read from an input file using four colons, parallel echo :::: params_file. This is particularly useful as it correctly deals with parameters/file names with spaces. For example, create a tab-delimited list of source and destination paths in paths.tsv and then run:
parallel --jobs 8 --colsep '\t' --progress rsync -avh {1} {2} :::: paths.tsv