Which could be the best way to admin OWB workspaces? for example: if we have 2 different providers and each of them is working on different projects:
1) Create 1 Workspace per provider, and give individual accesses to each?
2) Create as much workspaces as projects are, and give providers access to each workspaces/project??
The short answer is Yes.
The long answer is: Based on the little information you give, how could we know? It depends on your Business Rules, not on best practices.
For example there may be things you want your providers to share on a project, so a shared workspace for that project may be best. On other occasions you may not want them to share things, so you give them each their own workspace.