100 bananas gives you an extra life, but if you want to get more lives per continue, you need to complete some objectives. Some will give you more monkeys, and some are easier than others. Some examples are: beating Beginner (in Challenge Mode), getting 999+ bananas in Story Mode, or beating Story Mode. To get the most monkeys, you should play Expert (in Challenge Mode). It doesn't matter if you're not very good. I when I beat Beginner, I only got a couple of Monkeys (lives), but when I played Expert, I only got to Stage 6 (Exam-C) I got about 5 Monkeys. Hope I helped!