In addition to reducing the "the Minimum length of reas segments", do I also need to reduce "Anchor length" to get more mapping on splicing junctins?

Definitely worth a try.

Looks like the setting for "Anchor length" only affects the number of mapped splicing junctions reported in the .splicing junctions output. Is my understanding correct?

No, it will affect mapped reads as well.

Does the "regions" mean the number of mapped splicing junctions?