So far as I can tell, there is no plural form, although perhaps other Rad Ref librarians will come up with an example. I just checked the Oxford English Dictionary online (it requires a subscription, but your local library may have one), which says

Name given in South Africa to the segregation of the inhabitants of European descent from the non-European (Coloured or mixed, Bantu, Indian, etc.); applied also to any similar movement elsewhere; also, to other forms of racial separation (social, educational, etc.). Also fig. and attrib.

I would say that "apartheid policies" would be your best bet for a plural form, though it might be worth explaining to your audience that the term can apply to many different segregation policies, not just the South African one that we usually associate with the word.

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