You don't have to change a diaper a toddler or baby pees in the diaper.
That's just a waste of diapers and if you changed diapers every time the baby or toddler went pee then you'd go through a lot of diapers per day and you'd be doing not much more than changing diapers.
Disposable diapers are designed to absorb the urine and will keep the urine away from your babies skin so your baby will likely not even know they're wet after one pee.
Usually every 2 or 3 pees is when a diaper needs changed. You should check a diaper at least every 2 hours to see if it's wet enough.
The older a baby gets the less they pee and the longer the baby can go without having a diaper change.