In Jain cosmology, Tirthankaras are born in every half-cycle of time (kalpa). Each half-cycle (Avasarpini and Utsarpini) has exactly 24 Tirthankaras. We are currently in the Avasarpini (the descending half) and the 24 Tirthankaras of this half-cycle are already born, from Rishabhanatha up to Mahavira (the 24th). After this half-cycle ends, the next half-cycle (Utsarpini) will begin with a new set of 24 Tirthankaras. Digambar and Śvetāmbara traditions both affirm the same count, though they may differ in some historical details. You can read more here and here ."