Short answer Because of the fixed law of time in Jain cosmology: in every half‑cycle of time (each Avasarpini and each Utsarpini), exactly 24 souls attain the special Tirthankar-nāma‑karma and appear to re‑establish the “ford” (tirth) to liberation. So, per half‑cycle there are 24 Tirthankars—no more, no less.

A little more detail, kept simple

• Time is beginningless and moves in endless cycles with two halves: Utsarpini (ascending) and Avasarpini (descending). In each half, 24 Tirthankars manifest.
• They arise when beings especially need right guidance (mainly in the 3rd and 4th eras of a half‑cycle) and they establish the fourfold saṅgha: monks, nuns, laymen, laywomen.
• “Twenty‑four” is not a human decision; it is the eternal pattern described by the Omniscients and preserved in Jain scriptures. The cause is the extremely rare fruition of Tirthankar‑nāma‑karma, which occurs for 24 souls in each half‑cycle in our region.
• There are infinite half‑cycles and many regions; therefore, infinitely many Tirthankars across infinite time, but in our Bharata‑kshetra each half‑cycle always has exactly 24.
• Both Digambar and Shvetambar traditions fully agree on the fixed number 24. (They differ only in some historical details like Mallināth’s gender, not in the count.)

Today In our present Avasarpini of Bharata‑kshetra, the 24th and last Tirthankar was Bhagwan Mahāvīr. There will be no new Tirthankar here until the next half‑cycle begins.

For a simple overview, see: and a brief note on who a Tirthankar is: