Can the word "coexist" be used for more than two things/people/subjects/... etc?

Yes, the definition does not impose any limits on this:

Exist at the same time or in the same place.

(source: Lexico)

and there are some example sentences there as well which imply more than two subjects:

• Modern Western medical practices coexist with traditional healing methods and the use of medicinal plants.
• Nobody has convincingly explained how the birds, bees, flowers and hares of the uplands can coexist with a new influx of humans.
• Amazon plants, evergreen pines, basil and other strange flora coexisted in perfect harmony.

From wikitionary:

coexistence (countable and uncountable, plural coexistences): The state of two or more things existing together, usually in a temporal or spatial sense, with or without mutual interaction.

So yes, more than 2 subjects can coexist together.