For Castille/Spain multiple colonial nations is better, on top of the additional merchant, this allows you to go for extreme numbers of Treasure Fleet Gold.
By that I mean that all your CNs should be Crown Colonies with the "Enlarge the Gold Fleet" colonial relationship, which brings you a
stackable +20% treasure Fleet income.
https://eu4.paradoxwikis.com/Colonial_nation
Then you combine it with the other Treasure Fleet income modifiers, as well as Goods produced in CNs that Castille/Spain has plenty of:
- El Escorial great project in Madrid (+20%)
- Council of the Indies Tier 3 monarchy reform (+25%)
- Grand Armada Naval Doctrine (+50%)
- Exploration idea n°7 Global Empire (+25%)
- Exploitation of the new World Tier 7 monarchy reform (+10% goods produced in colonial subject, will affect the treasure fleet income)
- Zacatecas, Potosi, Tenochtitlan great projects-> A lot of goods produced modifiers, including in areas where multiple gold provinces might be.
I also took Infrastructure as the third Idea group (after Exploration/Expansion) to develop Gold provinces in CN to 10 prod as soon as possible, which impacts the treasure fleet income (and the policy with Exploration gives +1 dev after colony completion, also nice to have in an heavy colonial run)
I am doing such a run at the moment, with 31 colonial nations. The Gold Fleet from Mexico brings 4k+ gold alone (and then other CNs also bring a lot of money, I don't know what to do with).
Be aware that doing this will create a lot of Colonial border gore. I was able to keep it under control by releasing colonies, then conquering them again and feeding vassals so that they create their own CNs. Then annex that vassal and inherit the additional CN in a region I already had multiple ones. Asturias and León were great for this as they are relatively small and both have: +1 colonist in their national ideas and will have exploration if released post ADM 5.
I also tried to be somewhat historically accurate with the historical Spanish divisions in the Americas:
Colonial Mexico:
- Nueva Espana (main CN)
- Yucatan -> Yucatan peninsula, fed to Aragon before I took the decision to form Spain
- Guatemala -> provinces in Central America region but Mexico CN region, fed to Portugal before I annexed it
- Tejas -> Rio Grande region, fed to León. Sadly it colonized the Zacatecas region with its gold Great project, slightly reducing the treasure fleet income from Nueva Espana.
Colonial Caribbean
- Isla Juana/Cuba -> main colony
- Hispaniola -> fed to Aragon before Spain formation
- Antilhas -> Let Portugal colonize the rest of the islands, I had to feed Bermuda (technically not in a colonial region but in "greater Antilles" area with Jamaica) to the CN after annexation to get 10 provinces.
Colonial Brazil
- Ilha de Vera Cruz -> let Portugal colonize "Middile Brazil"
- Cabo de Consolación -> colonized first the North of Brazil, to get Tordesillas, released the colony (it became Brazil), then fed it to Aragon before forming Spain (source:
https://es.wikipedia.org/wiki/Cabo_de_Consolación)
- Misiones Orientales -> New Spanish colony I founded in the South of Brazil. It colonised a bit in the Plata region which I was fine with (
https://es.wikipedia.org/wiki/Misiones_Orientales)
Colonial Colombia
- Nicaragua -> part of the CN region in the North American continent, colonized first to get Tordesillas, released (it formed Colombia) then fed it to Aragon
- Venezuela -> main colony afterwards
- Tierra Firme -> fed native provinces to Asturias then let them colonize most of the region
- Las Guayanas -> Portugal colonized on its own and it took a couple of Brazilian colonial region provinces which I was fine with.
Colonial North America
- Florida -> fed native provinces to Aragon
- Las Carolinas -> fed native provinces to Portugal
- Axacan -> fed native provinces to Asturias, let them colonize the rest of the Southeast region (
https://en.wikipedia.org/wiki/Ajacan)
- Apalache -> main Spanish colony, with Sea access in West Florida area then following the Appalachian mountains (
https://en.wikipedia.org/wiki/Apalachee_Province)
Colonial Louisiana
- Luisiana -> fed native provinces to Portugal
- Pais de los Ilinueses (
https://en.wikipedia.org/wiki/Illinois_Country#:~:text=The Pays des Illinois is,the center of all descriptions.), accessed the provinces in the Great Lakes region/Lousiana CN region via the Apalache colony
Colonial La Plata:
- Chile -> main Spanish colony
- La Plata -> fed to Asturias
Colonial Peru
- Charcas -> main Spanish colony, Upper Peru region
- Peru -> rest of Colonial Peru region, fed to Asturias
Colonial Canada
- Lavrador -> founded first by expelling Basque minority, released the colony (it formed Canada) then fed to León
- Bacalaos -> (
https://en.wikipedia.org/wiki/Bacalao_(phantom_island)) in Nova Scotia and Newfoundland, split the former Lavrador CN provinces in two by feeding it to Asturias
- Borealia -> main Spanish colony in the Hudson Bay trade node region. Also expelled Basque minority there. All Canadian CNs are basque primary culture. First time I actually used the feature but for me it worked for roleplay reasons.
Colonial Australia
- Tierra Austral -> main Spanish colony in Australia
- Espíritu Santo -> fed New Zealand to Portugal, then fed Vanuatu/Fiji/Samoa to the CN to get the 10 provinces
For Colonial California and Colonial Cascadia (potential Spanish name "Territorio de Nutca") I didn't find a satisfactory way to split the CN and had already a lot on my plate thus the CN got the entirety of their respective regions. Obviously you don't have to do it but I wanted to push the concept as far as I could. For inspiration on the CNs I went to the Spanish Wikipedia page for "Imperio Espanol". They are quite generous with the land attributed to Spain/Portugal there:
