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?