In Mr. Monk and the Three Pies we meet Adrian's brother Ambrose. When Captain Stottlemeyer comes to Ambrose's house Ambrose gives him a cup of coffee with a number 6 on the mug.
Adrian explains that the number is to represent where the mug should go in the cupboard (between 5 and 7). From the transcript:
00:35:06 Why is it numbered?
00:35:07 Mother numbered ..
00:35:09 So we'd always put them back in the right order.
00:35:11 Of course she did.
Was Monk's mother also OCD, that she needed to have the mugs in a certain order, or was Adrian's and Ambrose's need for order such that she numbered the mugs so that her two sons wouldn't have to agonize about which mug went where?