I am in a graduate degree program and am trying to figure out the sample size, however, in order to figure out the sample size I must first figure

I am in a graduate degree program and am trying to figure out the sample size, however, in order to figure out the sample size I must first figure out my population sample, z score and margin of error. This is my current thesis study below.

What is the relationship between pre-medicating with multiple antiemetic agents prior to surgery and postoperative nausea and vomiting with the surgical patient?

Hypothesis Statement:

H0: Multiple antiemetic agents decreases postoperative nausea and vomiting

HA: Utilizing one antiemetic agent within the preoperative period does not decrease postoperative nausea and vomiting.

My instructor tells me this is a retrospective quantitative non-experimental study. And that I cannot "guess" the population size so do I go ahead and figure out how many surgical patients came through the hospital for one year? even though my data collection times are only a few months end of August 2017 through December 2017.