You asked

How often should I ask my child if he has to go to the toilet ?

Children and adults react negatively to constant nagging. And, sometimes when you ask your toddler over and over if they have to go to the toilet, it will begin to feel like nagging and they will start to resist. It’s difficult however, because you know you have to remind them to go potty. So, how do you do it without nagging?
Watch your child more closely for the physical signs that she has to go potty; facial expressions, grabbing herself, fidgeting, etc. If your toddler exhibits these signs, this is the proper time to ask. Eventually, your toddler gets the connection that when she feels this way, it is time to go potty.
If you notice that your toddler is becoming more resistant or telling you that she does not have to go potty even when it is obvious that she does, it could be that she is unconsciously balking the whole idea of potty training. This is definitely a sign that she is feeling too much pressure.
Perhaps this would be the time to take a break from potty training and try again in a couple of weeks.

More questions

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