The amount of diapers that an 18 month old uses a day is around 5 to 7 diapers although the amount of diapers can vary depending on how much the 18 month old eats and drinks.
Some days my 18 month old would go through only 5 diapers a day and other days when he drank a lot more he would go through 7 diapers a day.
Some days my 18 month old would poop 2 to 3 times a day and other times he would go a few days without pooping and needed less diapers per day.
Most 18 month olds will wet around 4 to 5 diapers per day although some may use more.
As children get older they start having less wet diapers and less poopy diapers during the day.
Sometimes I needed to change my 18 month old every 2 hours and sometimes he would go 3 to 4 hours without needing a diaper change.
Poop diapers I changed immediately but it's okay to let wet diapers go for awhile if you're using disposable diapers as they hold the pee away from the skin and keep them pretty dry.
Most 18 month olds also wear a size 4 diaper which is for children between 22 lbs to 37 lbs which is between the ages of 18 months to 3 years.
Many babies don't require size 4 diapers until they're almost 2 years old and many parents also don't stock up in the early days.
By age 2 a 2 year old often uses around 4 to 5 diapers a day and by age 3 they often use around 4 diapers a day but may need 5 diapers or 6 diapers a day depending on how much they drink and eat and how much they pee and poop.
Every child is different in how much they pee and poop and how often and how often they need a diaper change.