This triggered me to retest. Let me first apologize for calling you a liar. It's possible that you saw this.

I tried your formulation on the ChatGPT homepage in incognito mode (to rule out personalization for me). It said 2, so it's possible you saw that as well. I tried the same experiment again (new incognito mode, same phrasing) and it said 3.

That made me curious if phrasing made a difference and how often it would go wrong.

I tried 5 times with my own phrasing ("So, how many Rs in strawberry?") and got 5 times 3 as an answer.

I tried 5 times with your phrasing ("How many 'r's are in strawberry?") and also got 5 times 3 as an answer.

So 2 is an answer that can be given, but seems to be rare. At least in my tests.