If K'Sante, Dauntless is equipped with two Svellsongur and Nazuman Ntofo, how much shield does it have on defense?

Shield 40 while it's defending — so +40 Might, i.e. 44 Might total.

Stack the static Shield first: K'Sante has printed Shield 1, and each Svellsongur copies K'Sante's printed text onto itself, appending another Shield 1 to his rules text (same way copied Deflect stacks), plus Nazuman Ntofo's Shield 2. That's Shield 1 + 1 + 1 + 2 = Shield 5 before triggers (Rule 814.2).

Then the defend trigger: both Svellsongur also copy "When I attack or defend, double the Shield of a unit you control here this turn," so you get three separate instances (printed + two copies). They go on the Chain one at a time and each doubling is a snapshot add of the current value: 5 → 10 → 20 → 40. Not 5 × 2 × 2 × 2 read as one multiplication — but the result is the same 40 here. Note the Shield stays 40 for the turn, so if he defends in a second combat this turn those triggers fire again and it doubles further.