Hi,
You can use range validator. Add range validator dynamically in code behind file in combobox selection change event.
In selection change event method Add below code.
if(selected range is 1-5)
{
RangeValidator1.ControlToValidate = "TextboxId";
RangeValidator1.MaximumValue = "Give Max what you want";
RangeValidator1.MinimumValue = " Assign min what you want";
}
else if(selected range is 6-12)
{
RangeValidator1.ControlToValidate = "TextboxId";
RangeValidator1.MaximumValue = "Give Max what you want";
RangeValidator1.MinimumValue = " Assign min what you want";
}
Hope this might help you.