The literary device used in this sentence is hyperbole.
Explanation:
<u>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. </u>That is the case in the sentence "I love you so much I think of you a billion times a day." <u>What this speaker actually means is that he/she thinks a lot about a certain person. </u>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. <u>There is clear exaggeration here with the purpose of emphasizing the frequency of thoughts related to beloved person.</u>
