Use timer.schedule(), and keep track of how many times the timer was executed, and stop the timer after 20 times, with timer.cancel()

java doc - time schedule

Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals separated by the specified period.

In fixed-delay execution, each execution is scheduled relative to the actual execution time of the previous execution. If an execution is delayed for any reason (such as garbage collection or other background activity), subsequent executions will be delayed as well. In the long run, the frequency of execution will generally be slightly lower than the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).

Fixed-delay execution is appropriate for recurring activities that require "smoothness." In other words, it is appropriate for activities where it is more important to keep the frequency accurate in the short run than in the long run. This includes most animation tasks, such as blinking a cursor at regular intervals. It also includes tasks wherein regular activity is performed in response to human input, such as automatically repeating a character as long as a key is held down.

Parameters:

task - task to be scheduled.
delay - delay in milliseconds before task is to be executed.
period - time in milliseconds between successive task executions.

Example

Timer timer = new Timer();
int begin = 0;
int timeInterval = 1000;
timer.schedule(new TimerTask() {
  int counter = 0;
   @Override
   public void run() {
       //call the method
       counter++;
       if (counter >= 20){
         timer.cancel();
       }
   }
}, begin, timeInterval);

Try the Executor Service. You have to count youself, how often you called the Callable and cancel the Timer.

If it does not meet your requirements, you can follow this thread:

Java Timer to call function n times after every t seconds