Global variables don't trigger pattern matching so you shouldn't use them in the condition of a rule unless the value is never changed. If you use a fact to represent the lock, the rules will execute for as many cycles as you specify:
(deftemplate lock
(slot value))
(deftemplate counter
(slot number))
(deffacts initial_data
(counter (number 0))
(lock (value 0))
)
(defrule set_counter
?f <- (counter (number ?x))
?l <- (lock (value 0))
=>
(modify ?l (value 1))
(printout t "plus 1" crlf)
(modify ?f (number (+ ?x 1)))
)
(defrule show_result
?f <- (counter (number ?x))
?l <- (lock (value 1))
=>
(printout t "the counter value has been changed:" crlf)
(ppfact (fact-index ?f) t)
(modify ?l (value 0))
)