ADCRSA time of conversion.

If I want to do 30 conversion in 0,02s how I should code arduino ADC?
I know that conversion time is 1/(16MHZ(clock frequency)/13(clock tick before necessary to one conversion)) so normally for one conversion it is 81microseconds.

If I want to do 30 conversion in 0,02s how I should code arduino ADC?

Figure on each conversion using analogRead taking around 100us, so you should be able to manage around 200 analogReads in 20ms.

If you only need 30, then only do 30, with some pauses to pad-out the time.

, I would prefer not to use delay

Me too.
That's why I very carefully didn't write "delay"