PG Students must learn that good researchers cannot and will not do research based on convenience or the constraints an animal quotas or limited time opportunities – Good research is based upon and determined valid statistical analysis, so you need to conduct a power calculation to estimate your sample size
Work that is not statistically valid is not going to be published and to comply with the 3Rs you must only use animals for research that will produce statistically meaningful result (whether that result proves the null hypothesis or disproves the null hypothesis) otherwise the animals are wasted and that is against the philosophy of the 3Rs
https://www.stat.ubc.ca/~rollin/stats/ssize/n2.html
http://www.3rs-reduction.co.uk/html/6__power_and_sample_size.html
http://www.lasec.cuhk.edu.hk/sample-size-calculation.html
If you don’t know the variables for inputting into the calculators, then your literature search should be able to reference the variables for the calculator. If the variables required to plug into the calculator are not able to be found in the literature then a pilot study of N=3 -5 animals is justified to get a mean and SD.
Remember a pilot study needs IACUC approval