In Jainism, there is a single “first Tirthankara” in each cosmic half-cycle. The very first Tirthankara of our current time-cycle (the ascending half of the cosmic time, known as Utsarpini) is Rishabhanatha, also called Adinath.

There is no “next” first Tirthankara within this cycle. After the 24 Tirthankaras complete this cycle, a new cycle begins, and in that new cycle the first Tirthankara again will be Rishabhanatha. So the next time the role of “first Tirthankara” occurs, it would be in the next cosmic cycle and would be the same figure, Rishabhanatha, according to Jain tradition.

Note: some traditions emphasize minor differences in names or details, but the essential idea remains that Rishabhanatha is the first Tirthankara of this cycle, and the cycle will restart with him in the future.