Melbourne offers 330 places for Medicine each year. Monash and Melbourne both offer Graduate Medicine which requires you to complete any undergraduate degree as well as the GAMSAT. It doesn't matter which undergraduate degree and where you have completed the degree to be eligible. So you can complete a Science degree and Monash and apply to other Universities such as Melbourne and Deakin.
Monash also offer undergraduate medicine which you can go straight in if you meet all the requirements. If you are offered a place and you are sure medicine is the right choice, then go for it!
About the number of students getting in from Biomed or Science, I'm not sure about the exact figures. I would say a proportion of students from Biomed (and Science) will get in since not all in Biomed wants to get into Med, there are other courses such as Dentistry, Optometry, Physiotherapy and Research. But many students who meet the requirements have a good chance of getting in.