Autumn Brilliance can fruit alone, but it still benefits from a partner. Serviceberries can fruit alone, but two cultivars usually give heavier crops and better berry fill. It flowers in early bloom, which is worth knowing if you add a second tree later. Usually grown in USDA zones 4-8.
No single variety stands out as the obvious partner, but that is not a problem here: Northline, Rainbow Pillar, Regent, Smoky, and Thiessen all bloom close enough to work. There are 2 more in the list below. Pick whichever your nursery actually carries. Keep serviceberries close enough for early pollinators to move between them during bloom. A small orchard row or paired planting works well.
One thing to keep in mind. Commonly grown as an ornamental serviceberry, but it still fits as a practical flowering partner in mixed serviceberry plantings.
To put that into practice: at a minimum, one serviceberry can fruit, but two cultivars are a better plan if harvest matters. Better still, plant two to three fruiting serviceberry cultivars with overlapping bloom for heavier crops and better berry size. If you already have one ornamental-type serviceberry, add a fruiting cultivar before deciding the species is a poor producer.