When you create a custom OAuth2 helper script on a site you can use that helper script with any number of other sites. You are essentially creating a similar infrastructure as the one we provide on our own server. You can support multiple users per API application you create, which means that each of the sites using the same OAuth2 helper does not necessarily have to connect to the same storage account and storage space as the other sites. For example, two sites could be using your business' Google Drive, whereas another ten sites could be using each their owner's Google Drive account instead. The OAuth2 helper does NOT tie you to a specific storage user account or storage space. That's the whole point.
If you are a freelancer web developer, or a web development studio, you can provide your own OAuth2 helper scripts for all of your clients' sites. You do not, and should not, create a separate OAuth2 helper script for each and every site
Likewise, if you operate a number of sites for your own benefit, you do not have to, and should not, create a separate OAuth2 helper script for each and every site. You can set it up on one site, and use its provided OAuth2 helper URL on all of your other sites.
Keeping things centralised will make your life significantly easier. For starters, you only have to set up one API application with each storage provider you want to support across all sites you manage, lowering your upfront labor cost linearly with the number of sites you use it with. Moreover, when you need to do any kind of upkeep work on the API application – such as updating your business information, periodically confirming the validity of the information etc – you can do it once and be done with it.