I have 20 signals Cmos (10tx 10rx) to send...but i have a cable with only 11 pins.
With a standard serializer RS232 in need a cable with 21 does anyone can suggest me a idea?

so does anyone can suggest me a idea?

Yes - produce a clear specification of what you wish to achieve, in terms of performance and resources available, not some word soup.

The signals (any tx -rx) are sent with baud rate 57600.
I need remains with this baud rate

I should think that the number of pins on the plug is the least of your worries if you're simultaneously sending and receiving 10 Serial signals. You would need an Arduino with 10 Serial ports, which doesn't exist.

The Teensy 3.6 is the best Arduino compatible with 6 Serial ports. Even then, it's going to be a challenge to get two of them working together to do whatever you think you need to do.

Use two or more cables in parallel. Each carries 5 signals.