If the Invaders are entirely prevented from Exploring on a board (whether due to lacking sources near the affected lands or due to those actions being skipped) then they don't add an extra Explorer there. If there are multiple Explore Cards during a single turn, the effect happens for each of them. This does affect both Explore Cards resolved in setup, but doesn't affect Explore Actions caused some other way (e.g., the "Search for Unclaimed Land" Event or the Escalation of Habsburg Mining Expedition).
