put the whole code in a while True loop and at the end ask the user if they want to repeat. if not break the loop.
something like this:
while True:
sentence=input("Please enter sentence(s)")
words = sentence.split()
number_of_words = len(words)
counter=0
for x in sentence:
if x in "!?.":
counter=counter+1
print("There is "+str(counter)+" sentences and " + str(number_of_words) + "words")
if input('Do you want to repeat(y/n)') == 'n':
break
