Not sure what you mean by "after specific item number" -- I assume you mean after X amount of items (e.g. half). What you do is create a variable (a <values> entry) to count the trials.
/ trialcount = 0
Increase it by one in /ontrialbegin in <trial getstim>
/ ontrialbegin = [values.trialcount += 1]
Then create a "break" trial
/ stimulsframes = [1=breaktext]
/ validresponse = (57)
/ branch = [trial.getstim]
/ items = ("Take a break")
and then /branch to the break trial as needed from <trial mwstim>
/ ontrialbegin = [expressions.movingwindow]
/ stimulusframes = [1=background, mywindow, mywindowhl]
/ validresponse = (57)/ branch = [if(values.winend==expressions.inputend && values.trialcount == 5)trial.breaktrial]
/ branch = [if(values.winend==expressions.inputend)trial.getstim else trial.MWtrial]
The above would run a break after 5 trials.