One meta-misunderstanding - a bit like the false claim that the Europe of his day thought the earth was flat - is that Columbus thought he had reached India in the sense we usually mean historically (broadly, South Asia), rather than the Indes. He didn’t think he had reached the likes of Tamil Nadu or Bengal, but mistook the Taíno and (Island) Carib people of the Caribbean islands for the inhabitants of the eastern ‘spice islands’, or others nearby, mainly of what we’d now call Indonesia. Just as ‘India’ or its cognates was used as a general term by the Persians for the land ‘across the Indus’, the ‘Indes’ was used for islands east of the Indus. The word ‘indios’, translated ‘Indians’ was used fairly carelessly for both or all such contexts, but this doesn’t mean they thought they were the same.

It wasn’t very long that the West was under this misapprehension, even if the name stuck for a very long time: Columbus’ four voyages spanned 1492 to 1504. During that time, three other European explorers reached Newfoundland and/or Labrador (John Cabot - apparently twice, João Fernandes, and the brothers Corte-Real), and five others reached the northern parts of South America (Alonso de Ojeda, Amerigo Vespucci apparently twice, Pinzón, Cabral, and de Bastidas). Most accounts of these either referred to the Indes or stuck to more specific geography (eg, of the specific rivers and islands themselves, or giving a region of mainland Brazil a misconceived name like ‘Island of the True Cross’… or about the peninsulas or islands of what appear to be Newfoundland and Labrador) without a clear picture of how they thought these lands related to the wider world map already know to the east.

We have two somewhat controversial accounts that claim to be by Vespucci from 1503 and 1505, about his 1499 and 1501 expeditions. In them the author claims to have realised that where he landed in what we would now call South America was part of a whole separate continent.

It’s also fair to note that most scholars of the day had a reasonably good idea of the size of the earth, based on Eratosthenes’ ancient but surprisingly accurate calculations (there is still discussion about exactly which Greek convention of units he was using, and that he may have had had two errors cancel each other out), and navigators were aware of the distance and direction they had travelled. Ptolemy’s estimate was much smaller, and there was a spread of estimates popular at the time, but Columbus’ view was a minority one distinctly on the shorter end than what most of his contemporaries agreed with, about 70% of the true value. He also took an overly optimistic estimate of the distances involved from by the mathematician Toscanelli.

Other Italians from Marco Polo to Niccolò de’Conti has made journeys to India and even China, so there was indeed awareness of the differences between India as such and the islands Columbus reached.

So none at any point clearly thought they had reached ‘India’ in the usual sense. Of the first 15 or so European voyages to America within the first 11 years of Columbus’ first, it is unclear what most of them thought, but many scholars would have been able to deduce that the Americas were far, far nearer than the ‘spice islands’ and by the end of those 11 years Vespucci’s widely-read alleged account makes the realisation they’d discovered a ‘new’ continent explicit, and after Waldseemüller wrote his famous map in 1507 (showing the Americas as quite distinct from Asia, and calling South America ‘AMERICA’) this became widespread.