A common source of confusion for some English learners is the use of the numeral one vs. the indefinite article a or an. The tendency is to use one rather than a or an, which sounds unnatural in English.
The reality is that native English speakers don’t use one much, even when they refer to a number. When there is just one thing to count they basically ignore the numbers and simply use the indefinite article a or an, for example:
“I’ll have a pint of Guiness, a gin and tonic and two Cokes, please.” (notice that for Cokes the number was used – two – since the person wanted more than one)
“He was away for a week.”
“She’ll be back in a minute.”
“We’re closing the office for a day.”
If you want to make yourself clear, and emphasize a single item, use the numeral one, as in: