Johns can fruit alone, but it still benefits from a partner. Elderberries often crop better and more heavily when two cultivars bloom together. It flowers in mid bloom, which is worth knowing if you add a second tree later. Usually grown in USDA zones 3-9.
No single variety stands out as the obvious partner, but that is not a problem here: Adams, Marge, Nova, Ranch, and Scotia all bloom close enough to work. There are 4 more in the list below. Pick whichever your nursery actually carries. Keep elderberries in the same row or nearby rows so pollinators can move easily among the flower clusters.
To put that into practice: at a minimum, one elderberry can fruit, but planting two cultivars is the better baseline. Better still, plant two to three elderberry cultivars with overlapping bloom so crop size and berry fill stay strong. If you already have one elderberry, add a second named cultivar before deciding the original plant is unproductive.