I think that in Polish it rather means just one mug.
A more natural thing would be to simply say "przynosi nam herbatÄ™", which is treated as a mass noun here and doesn't need to be pluralized to mean that everyone gets their own tea.
I wrote " My mom is bringing a cup of tea." and it told that it was wrong and when I looked at the real answer it wrote "My mom brings us a mug of tea." like what is difference? ????