As specificagent1 mentioned, you can achieve a 99+ ATAR with any combination of subjects. What may vary is the difficulty of achieving such an ATAR with different subject combinations. Additionally, your determination and diligence relevant to your studies are preparations are also crucial in ensuring optimal performance in your assessment tasks/exams and ultimately maximising your chances of achieving your desired ATAR.
To further support the notion that a 99+ ATAR is achievable with any combination of subjects, and with respect to your question, a 99 ATAR with Mathematics Standard 2 is definitely possible. Looking at the latest data from UAC, the highest ATAR that was achieved by a student who took Mathematics Standard 2 in 2020 was 99.85.
I hope this helps!