Crucial to understanding this excellent sonnet revolves around your appreciation of how the ideas expressed in this sonnet are structured. The sonnet begins with a question in the first line that is responded to by negative answers. Whilst the speaker's beloved does bear some resemblance to a "summer's day," they are but superficial, and the first two quatrains focus on the ways that the summer day is not perfect rather than the loved one. Then in line 9 we have the turn, or "volta," when the speaker focuses on his beloved and forgets the summer's day.
So, when we think about the figures of speech, they are either employed to describe the imperfections of the summer's day or the speaker's beloved, build around the extended metaphor of comparing the beloved to a summer's day and finding the summer's day wanting:
But thy eternal summer shall not fade,
Nor lose possession of that fair thou owest...
We can see here that continuing the extended metaphor, the speaker explains the way that his beloved is actually better than the summer's day he wants to compare her to at the beginning, saying that her "eternal summer" will not diminish and her beauty will not decline. Hopefully this description of the sonnet will help you pick out other figures of speech. Good luck!
As many as you can count. Take into account that every way of the word that differs from common conversation speech can be considered a figure of speech. On the other hand, usual speech in a literary work should also draw our attention just because it has not been alterd.