What's the danger of narcosis with a tonsilectomy?

Minimal. If one takes pain medication appropriately the dosage should not cause a stupor or narcosis. If you take more than the prescribed dose you will have a significantly increased risk of narcosis.
Narcosis post tonsil. There is minimal inherent danger of narcosis with a tonsillectomy. Some ent's will prescribe medications with narcotics for pain control after tonsillectomy. If these medications are taken as prescribed, there is very minimal risk of narcosis.
A lot. Narcosis is when one's CO2 level increase to the point of obtundation because of decreased ventilation. To reverse this a patient would have to be bag ventilated and possibly re-intubated. Doing this could then disrupt any hematoma resulting in massive bleeding requiring being taken back to surgery. This is an emergency.