Volume of the mouth is about 150mls (half a can of coke). Maybe you have a big mouth so let's call it 180mls. That's 10 moles of water (assuming saliva is 100% water). At 0.9688gcm^-3 they're basically 1:1 and the stoichiometry is also 1. So max you can fit about 115g of sodium to 90ml of water.
If we ignore the dynamics of the explosion that seems enough to cause a maybe lethal explosion. It would also release 120dm^3 of hydrogen.
If we ignore the dynamics of the explosion that seems enough to cause a maybe lethal explosion. It would also release 120dm^3 of hydrogen.