Rate limits — how many requests per minute are allowed?

60 requests per minute per API key. If you exceed it you'll get a 429 response with a Retry-After header telling you how many seconds to wait. For anything involving more than a handful of records, using the bulk endpoints (contacts, unsubscribes) where available is the better approach — they count as one request regardless of how many records are in the payload, up to 500 per call.