I love you so much I think of you a billion times a day which literary device is used in this sentence

The literary device used in this sentence is hyperbole.

Explanation:

Hyperbole is a literary  as well as rhetorical device that uses exaggeration to express a feeling, an action, or an idea. The purpose is to inspire strong feelings or impressions in one's audience. That is the case in the sentence "I love you so much I think of you a billion times a day." What this speaker actually means is that he/she thinks a lot about a certain person. However, not only is it impossible to know how often you think of someone, it is also impossible to think of anything a billion times in just one day. There is clear exaggeration here with the purpose of emphasizing the frequency of thoughts related to beloved person.