There is option --max-multimaps INT, which makes Giraffe output at most INT alignments for each read. We don't really test it, but it should work. The actual number depends on how many candidate alignments pass earlier filters, such as --max-alignments INT (do tail alignment for this many extended seeds) and --extension-score INT / --extension-set FLOAT (thresholds for doing tail alignment based on scores relative to the best score).

You can find all the hidden options with vg giraffe -h. Most computational parameters can be adjusted, but there is little documentation for them and changes may have unexpected consequences.