Can you say “you can send letter to her address” without specifying singular or plural? Or can you say: you can send a letter to her address and you can send more than one time. Because if I say send letters, that might sound like send multiple letters at a time

Question

Can you say “you can send letter to her address” without specifying singular or plural? Or can you say: you can send a letter to her address and you can send more than one time.  Because if I say send letters, that might sound like send multiple letters at a time

Answer

It isn't necessary to specify singular or plural unless there is a specific context.

I'm not too sure of the exact context/situation which you may be referring to, so I provided  context in my response.

If it's just a general suggestion...you can probably say something like:

"You can always send a letter to her address if you ever need to reach out to her."

This would imply...that you can send ONE or MORE letters to the address...as many as needed/required.

Similarly:

"You can always send letters (plural) to her address..."

...does not necessarily mean they're expecting you to send more than one...it's just a general suggestion.