Are there any numbers for the amount of immigration from Castille to the New World?
And on the side, are there any circumstances that could see Spain allowing for Catalans and Italians to migrate, or even people from the Netherlands, Austria and Germany? I can imagine a situation with a more successful Reformation(in terms of politics, not necessarily converts) that sees a lot of displaced Catholics, but I'm not sure the Spanish would (ever) be that magnanimous.
Regarding the numbers if castilians (not sure if you ask about castilians properly or subjects of the castilian crown, which would include basques, galicians, andalusians etc) there are estimates, but I don't have them at hand.
Regarding catalans, like other subjects of the aragonese crown (Aragon, Catalonia, Valencia and the Balearics), they were allowed to migrate and in fact they migrated. Even in first Columbus voyage, the Catholic Kings' representative was a catalan, Pere de Margarit. However they weren't allowed to trade till the Nueva Planta decrees in 1714. Catalans migrated in large numbers to the Antilles and the Philipines in the late 18th century an 19th century. Famous rum labels like Bacardí or Barceló were founded by catalans, for example.
Now, the problems to somebody who wanted to migrate to the Americas where two. One was the royal permit, and the other the geographical accessibility. The first waves of colonists were mainly from south western areas because the only port allowed to trans-oceanic travel at the time was Seville. The crown wanted to assure its revenues. If you look at a map of Spain you will understand it quickly.
Of course, there were also other side factors. Extremadura, being rather poor, was over-represented, thus Cortés, Pizarro, Núñez de Balboa, Cabeza de Vaca or Valdivia, to name only some, were extremenians. Though you have also conquistadors and settlers from other places, like Aguirre, who was basque or Ponce de León, who was castilian (sensu strictro), the number of extremenians, considering its low population, is very high in the 1500's. In absolute numbers, in this period the andalusians were dominant, for obvious reasons.
When ports in Galicia and Cantabria were opened to american trade in the 18th century, galicians and basques became the most numerous migrants. It was also helped by the peculiar basque inheritance laws and a shortage of land in Galicia.
For a catalan or a valencian, then, in order to emigrate to the Americas in the 1500's and 1600's, it was necessary to make a long trip to Seville and then hope for the bureaucrats to be quick and grant you a licence to travel. Meanwhile Italy, the mediterranean trade or the campaings against the Ottomans and the Barbary pirates were next door, either for profit or adventure.
Regarding italians, tbh, I'm not sure, since technically they were aragonese subjects. Anyway, the problems faced by aragonese, catalans and valencias would be even greatr for them, as the incentives for other options. Go figure for flemish or germans, who also spoke totally ununderstable languages (in relation to castilian) Still, there were plans for a "german" settlement in OTL venezuelan coast, but vested interests and pressures by pearl collectors in the area, amongst other problems, left the idea only in the paper.
I hope I have solved at least some of your doubts.