Short answer:

• In our current half–time cycle (Avasarpini) of Bharat Kshetra, exactly one Chovisi (set of 24 Tirthankaras) has occurred, and it is complete. The 24th was Bhagwan Mahavir.
• Across the endless past and future cycles of time, the number of Chovisis is countless (anant).

Helpful context:

• “Chovisi” means a group of 24 Tirthankaras. Every half-cycle (Avasarpini or Utsarpini) has one Chovisi in Bharat and in Airavat Kshetra. So per full time cycle there are two Chovisis in each of those lands.
• Both Digambar and Shvetambar traditions agree on “24 per half-cycle.” (They differ on certain historical dates, not on the count.)

For a simple overview of the Jain time-cycle and how Chovisi repeats: