To 'take something with a pinch of salt/grains of salt' means to take something with a degree of scepticism or not to take something literally. So to take something with a million grains of salt expresses the same sentiment but much stronger.

If you say, "I'll take that with a grain of salt", it means that you are somewhat skeptical about what someone else is telling you. I'll take that will a million grains of salt is a mildly humorous way of saying that you are VERY skeptical. People do not normally say 'a million grains' - it's just a silly way to exaggerate the original saying. Hope that makes sense.

To take something with a grain of salt means that you should do it, but be a little bit skeptical or suspicious. Taking it with a million grains of salt means that the person is taking it even they know for sure that they should be suspicious.

Thank you, Matt G :)

Taking something with a grain of salt means you know it might not be true, or it might not be exactly true; the source of the information isn't reliable. So taking something with a million grains of salt would mean the information is might be true, but probably isn't. "He said the other person cheated, but he always has an excuse for losing, so take that with a grain of salt." The addition of "million" isn't common, but doesn't change the underlying meaning, only the degree of reliability.