It is difficult to provide an exact number or statistic for the specific behavior you mentioned as it is considered a niche or fetishistic practice. Treating a partner as a baby with nappies, plastic pants, dummies, cots, and other infantilizing items falls under the umbrella of a fetish known as "adult baby/diaper lover" (ABDL).
ABDL is not a common or mainstream practice in relationships, and its prevalence is challenging to measure accurately due to its private nature. It's essential to understand that people's interests, desires, and kinks can vary widely, and what might be appealing to one person may not be for another.
If you are interested in exploring this topic further, it's essential to do so with open communication, trust, and mutual consent with your partner(s). It's crucial to ensure that all parties involved are comfortable and willing participants in any activity or roleplay in the context of a relationship. Additionally, if you or someone you know is struggling with ABDL tendencies or any fetish that may be causing distress or impairing daily functioning, it might be helpful to seek guidance from a mental health professional or therapist.