To my knowledge, there is no hard limit to the number of criticals that a weapon can do. Some of the artifacts in the Lords of Middle Earth volumes, IIRC, did tons of extra criticals.
Slaying criticals are extra criticals that are rolled in addition to the normal (e.g. slashing, crush) critical of the weapon. This is from Arms Law & Claw Law (red band edition, #1100, section 6.4.2, pp. 18-19). The text specifies what Ginger McMurray noted: that slaying criticals against 'man-sized' creatures are rolled in addition to the normal critical.
What the rules don't seem to specify is whether this rule also applies to Holy, Mithril, or other types of criticals that weapons can do. We assumed that the same rule applies to them, and this makes more sense to me. I say this because if it were not the case, low rolls with Mithril/Holy weapons would be worse than normal criticals. If you take a look at the Large Creature critical chart, rolls as high as 50 produce just extra hits damage, but a 50 on an E Slash would stun foe two rounds and result in an injury (-15 penalty). It doesn't make much sense to me if a Holy weapon did less damage than a normal weapon.
As for other types of criticals such as fire, sometimes the weapon will specify whether it does 'extra' criticals or not. In the original Creatures and Treasures, for example, p. 73, the weapon Noril is said to do 'an addtional' heat critical. In that case, I think it is clear that the weapon is supposed to do a normal critical and an additional one on top. When it is not specified, I think it is a case of GM judgment.