Timer problem..!!

I want to turn a motor on when a count reaches 20. The motor is then to go
off when a count of 30 is reached.
Motor becomes on-- when count reaches to 20, But how will motor become off when it reaches 30..!! As the objects are not uniformly coming, timer can't be used.!!

The answer resides within your inquiry; you stated "when a count reaches..." thus you should be using a COUNTER and not a Timer.
I believe COUNTER instructions are available for pretty much any available PLC platform.
"Count UP" any event to be monitored; when the Counter Accumulator equals one of the benchmarks (20 in your case) make the "logic decision" (turn on the PLC Output controlling the motor starter); keep the "decision" true when the Accumulator is Equal OR Higher than the first benchmark but Lower than the second benchmark (30 in your case); when the Counter Accumulator is Equal OR Higher than the second benchmark, "cancel the logic decision"(turn off the PLC Output controlling the motor starter).
Don't forget to RESET the Counter Accumulator at the end of the cycle.

I think you can do very well with timer also...
Before starting something, think how to stop it!
Below example with timer:
Regards.