Max available serial (RS232) for STM32 Eval boards is two.
You can use ST-MCU-FINDER to explore the STM32 portfolio and find product that will fit your needs.
For example, you can search products in "Board Selector" with update value in RS-232.
Note that ST-MCU-FINDER feature is included in the STM32CubeIDE and STM32CubeMX tools.
Best Regards,
Imen
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RS232 is a bit dated, most applications today use MCU to MCU level connectivity at CMOS levels.
I would recommend using a NUCLEO-144 board, finding a handful of UART pairs, and then wire in simple RS232 DB9 w/MAX3232 adapters from eBay/AliExpress or you're local vendor of choice.
The OP specifically asked for RS232, presumably because he had existing legacy equipment using that mode of connectivity.. ie loggers, control or telemetry equipment
I can see a custom board having multiple RS232 ports, pretty deep into a niche application for it to be on mass-market evaluation board.
Many flight controller boards could also provide multiple UARTs
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