Titania can fruit alone, but it still benefits from a partner. Currants often fruit alone, but a second cultivar from the same currant group usually improves yield. It flowers in mid bloom, which is worth knowing if you add a second tree later. Usually grown in USDA zones 3-7.
No single variety stands out as the obvious partner, but that is not a problem here: Crandall, Ben Sarek, and Consort all bloom close enough to work. Pick whichever your nursery actually carries. Currants are small shrubs, so keeping them in the same bed or nearby rows makes pollination easy.
One thing to keep in mind. Red and white currants are best matched with each other. Black currants are best matched with black currants.
To put that into practice: at a minimum, one currant can fruit, but two cultivars are the better plan if you want stronger harvests. Better still, plant two or more currants from the same currant group so bloom overlap and fruit set stay reliable. If you only have one currant, add a second cultivar from the same currant group before assuming the first plant is unproductive.