Dropbox Finder Extension spawning multiple processes in Activity Monitor is a normal and expected behavior of Finder Sync Extensions in macOS.
Nothing to do with Dropbox, really. All Finder Sync Extensions (Dropbox, OneDrive, Google Drive, Creative Cloud, etc.) will all run multiple copies of the process as directed by Finder and macOS. The app itself can't stop that behavior.
Apple documentation:
Performance Concerns
Finder Sync extensions may have a much longer lifespan than most other extensions. Because of this long lifespan, you must take particular care to avoid any possible performance issues. Ideally, Finder Sync extensions should spend most of their time running but idle. Limit the number of resources the extension consumes. Most important, be sure to avoid leaking any resources. Over time, even a small trickle can grow into a serious problem.
The system may also launch additional copies of your extension whenever an Open or Save dialog is displayed. This means that the user may have multiple copies of your extension running at once, and some may be very short lived. Therefore, it’s generally best if the extension focuses on handling the badges, contextual menus, and toolbar buttons. Place in a separate service (a Login Item or Launch Agent) any code that performs the sync, updates state, or communicates with remote data sources. This approach ensures that there is only one syncing service running at a time.
https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/Finder....
Each Dropbox Finder Extension process should be using minimal resources, and if they are not, that is a performance concern.
But multiple Finder Sync Extension processes, using minimal resources, are normal and something that you will see in every single Finder Sync Extension on your Mac.
