Creating bulk Google accounts on only one device seems not possible since it has many limit such as proxy and phone number.
You must use a tool like PVAcreator which help to solve those kind of problem automatically.
- It has integrated all kinds of SMS service for users to get the otp automatically.
- It allow users to add proxy for each account thread creating. In this way, can it keep your accounts safe.
- Moreover, it has account warm up function. You can also to keep different accounts in different browser.
- The best part is the anti-detection fingerprint browser.