"Do you believe that mercury is toxic?"
As with all things, mercury is toxic at the right dosage.
"How many micrograms/day is a toxic dose?"
I am still trying to understand this myself. It may depend on variables such as route of absorption (food & Amalgam = GI, vapors = inhaled) and the body's ability to clear it (renal function, etc). The WHO recommends a maximum of 25 ug/m3 in the air, but this doesn't really address how much is actually absorbed into the body.
"How many micrograms/day is one exposed to if one has ten or more fillings?"
Extremely variable, depending on things like number of teeth, number of surfaces, baseline mercury release, magnification factors such as eating or toothbrushing, eating habits, oral breathing habits, nose-mouth breathing ratio, inspiration-expiration ratio, swallowing, inhalation absorption, ingestion absorption and body weight. It seems most studies consistently find mercury release rates around 1 to 2 µg/day for subjects with more than eight Amalgam restorations, although several researchers have arrived at figures higher than 10 µg Hg/m3. This is still below the WHO recommendations.
Olsson S, Bergman M. Daily dose calculations from measurements
of intra-oral mercury vapor. J Dent Res 1992;71(2):414-23.