1. I have acne.

2. I have an acne.
3. I have acnes.
4. I have pimples.
5. I have pimple.
6. I have a pimple.
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Is 'acne' the same as 'pimple'?
Which ones are grammatical? Can we substitute 'pimple' for 'acne'?

2 answers

1, 4, and 6 are correct.

A pimple is one blemish. Acne means a lot of such blemishes.
Acne is not a pimple acne is a whole bunch of pimples/blemishes.