Santa Rosa is partially self fruitful, but it produces better with another variety to pollinate it. For this year I suggest you search for another plum variety that is blooming at a neighbor's or friend's house. Cut a few branches and place them in a bucket or coffee can with some water in it. Then set the bucket under the tree or tie it up in the branches. Bees will visit those blooms and carry pollen to your tree's blooms. Replace the cut branches after about 3 days with fresh ones.
For a long term fix, either graft or bud another plum variety into your tree, or plant another variety nearby.
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