Slightly surprisingly, “advice” is an uncountable (mass) noun in English (like “water” or “sand”), and as such it has no plural form:
correct His advice was very helpful.
wrong His advices were very helpful.
Thus, we speak about the amount of advice, not the “number of advices”:
correct I didn’t receive much advice.
wrong I didn’t receive many advices.
Since it is uncountable, we cannot say “an advice”. We would usually say just “advice” (without an article), or, if it is necessary to emphasize that we think about it as about one piece of information, we use “piece of advice”:
correct This was good advice.
correct This was a good piece of advice.
wrong This was a good advice.