A few is a positive term that indicates a small number. When we use it we are asserting the positive existence of a small number of something.
Few is a negative word. It has a negative meaning. It means not many. If we use few, we are stressing there were not many, when perhaps people thought there were.
If someone offers you a job and you ask:
- Are there any opportunities for promotion?
Then if they say:
- There are a few opportunities for promotion.
This means that the speaker thinks that you may have a chance to get promoted. However, if they say:
- There are few opportunities for promotion.
Then this means that they are stressing that there are not many opportunities. They probably think it is unlikely you will ever get a promotion.
We can show that a few is a positive term and few is a negative term by putting a question tag onto each sentence. Usually, if a sentence is positive, it will have a negative question tag. If a sentence is negative it will have a positive question tag:
- There are opportunities for promotion, aren't there?
- There are a few opportunities for promotion, aren't there?
- There aren't opportunities for promotion, are there?
- There are few opportunities for promotion, are there?
From the examples above we can see, that a few does not change the polarity of the sentence. The sentence remains positive. In contrast few makes the sentence negative.
The Original Poster's question
The Original Poster probably wants to say:
- I took a few deep breaths
This is because they just want to explain that they took a small number of deep breaths. They probably didn't want to say that they didn't take many breaths. There is no reason to emphasise NOT MANY in this situation.
Hope this is helpful!