Q.Planting desert willow seeds
I have some seeds from the desert willow pods. Do I plant one seed or several seeds together? Can I do anything to prevent having several trunks? I have seen a tree that has about a 5 inch diameter trunk and it looks better to me than having several trunks.

Yes, you just need to select a single trunk. These links will help you.
https://www.gardeningknowhow.com/ornamental/trees/desert-willow/pruning-desert-willows.htm
https://www.gardeningknowhow.com/ornamental/trees/desert-willow/caring-for-desert-willow-trees.htm
https://plants.usda.gov/plantguide/pdf/pg_chli2.pdf