The writing's on the wall -- or the body -- for the downfall of Mauricio Umansky's relationship with Kyle Richards ... 'cause the guy forgot how many tattoos his wife's got.

Kyle informed Mauricio about her tat count in a preview for the upcoming season of "The Real Housewives of Beverly Hills," which premieres Wednesday night on Bravo -- while he was convinced Kyle only had 3 pieces on her, he forgot about 2 others.

She even threw a playful jab at him when tattoo numbers 4 and 5 slipped his mind ... telling him he should be paying a bit more attention to her body.