In Jain cosmology, there are 2 chovisīs in one complete time cycle:

• 1 chovisi in Avasarpiṇī
• 1 chovisi in Utsarpiṇī

So, in one full Kalachakra, there are 48 Tirthankars in total.

Time frame

A chovisi means the period in which 24 Tirthankars appear. These 24 Tirthankars are generally spread across the 3rd and 4th aras of that half-time cycle.

In short

• 1 half-cycle = 1 chovisi = 24 Tirthankars
• 1 full cycle = 2 chovisīs = 48 Tirthankars

If you want, I can also explain the 6 aras of each half-cycle in simple terms.