The AdBlue tank typically takes around ~20-22L, which should last for around 10,000 miles of driving. It is being topped-up during servicing, and unless the car does a lot of miles between services, owners don't usually need to fill-up with AdBlue themselves.
(BTW, I believe that the AdBlue tank is also being completely emptied and refilled every other service)
That said, and not wishing to cause undue alarm, a failed sensor is a common fault that sadly may nesseciate the replacement of the entire AdBlue tank unit. Given that your car is still within the 6 months statutory warranty period, I would take it back to the dealer it was bought from, and ask them to top it up for you. They might charge a bit more, but you'll get the reassurance of knowing that if anything turns out to be a miss, they'll need to fix it FOC.