Hello Tharane; Your sample includes those specimens that you actually counted. So if, for example, your traps contained 142 mice after a week, your sample size was 142.
You may be intending to determine the population size of the study site. To calculate that you will have to employ some "mark, recapture" method.
If your study compares groups, 10-20 animals per group is acceptable. This number can balance logistical convenience and data collection to guide the full-scale study design.