There is no fixed amount of math you should expect in a Bain interview. Some cases may have one major quantitative section, while others can include several smaller calculations as the analysis develops.
The better way to prepare is to assume that you need reliable quantitative skills throughout the case. You should be comfortable setting up calculations, working with percentages and ratios, interpreting charts, and explaining what the result means.
Bain interviewers are not looking for a human calculator. They are looking for accurate, business-relevant quantitative reasoning. A slower candidate who sets up the math clearly and interprets it well can perform better than someone who rushes to a number and makes avoidable mistakes.
So I would strengthen the core math skills, but do not let fear of math dominate your prep. It is one component of the overall case.