When to use plural of "archipelago"

The definition of archipelago by Oxford Dictionary is the following:

an extensive group of islands

The definition of archipelago given by Cambridge Dictionary is instead the following one:

a group of small islands or an area of sea in which there are many small islands

I can conclude that, by both definitions, archipelago itself always means plural.

Both dictionaries give us the plural of the word as well, which are archipelagos and archipelagoes.

When should we use archipelagos or archipelagoes which means the plural of an extensive group of islands?

Archipelago is a singular noun as the group of islands is viewed as a collective whole. Archipelagos or archipelagoes is the plural noun referring to more than one group of islands.

Your two dictionaries say that an archipelago is an extensive group of islands or a group of small islands. As Jasper Loy says, collective nouns are singular by definition: they refer to a group of things.

Here's another example of plural use: There are three archipelagos off the north and north-west coasts of Scotland: the Orkney Islands, the Shetland Isles and the Western Isles (or Outer Hebrides).