No-one knows, really.

If it comes to court, there are 'expert' witnesses who will testify that an arrangement is 'recognisably the same' but as far as I know, there is no scientific measure of how similar two arrangements are (or two tunes, for that matter).

It may depend on a variety of things, including chord sequences, time signature, or individual notes in a similar pattern.

Compare Paul Simon's Scarborough Fair with Martin Carthy's version.
I believe Paul Simon's was held to infringe Carthy's copyright.