When Omega2 is in AP mode,

What is the maximum number of stations which can be connected to the Omega2 Access Point at the same time ?

In general, other wifi module's softAP limits the station number. For example, espressif's esp32 AP can be connected only four staions at the same time.

Omeag2 AP has a limitation of connectable stations ?

The situation may be more hopeful than with the ESP series though - those are really designed to be a tiny gadget and function mostly in the station role. Conversely, while the Omega2 is a tiny gadget, the heritage of its MT7688 SoC/WiFi and its LEDE(OpenWRT) software stack are to things intended for small home/travel routers, so in their original use would probably support a "reasonable" number of clients. The 2+ especially even has more memory than a typical budget router would. But the stock software might or might not be configured to do this well.

DOUBLE POST! The conversation should resume here:
https://community.onion.io/topic/1061/access-point-questions

I have read this conversation before posting. At that time, I didn't catach the information.
Now, I read carefully again, it is clarified that "omeag 2 (openWRT) AP's client limitation is 253."
Thank you for your reminding.