It is suggested that chances to see through a veil are determined by the number of units veiled multiplied by the number of warlords looking. In Book 1 page 107 there are 11 warlords looking for "30 or so" units giving around 300 chances. A direct multiplication would give 330 chances for that particular stack to find 30 veiled units.Erf-b1-p107
Another explanation for "330 chances" is that each warlord rolls once against each veiled unit that it might see. If the roll is a success, that warlord sees that unit. The chances of winning that roll would therefore still be unknown.