How can accountants who are serving clients, sign up to use Wave?
You can just sign up to Wave! If your clients are using Wave and want to add you to their business as a collaborator, they'll have to go to their Settings page and click on "Collaborator".
They'll add your information there and you'll receive an email containing a link to accept that collaboration request.
How many clients can you have add you as a collaborator?
There are no limits to the number of businesses you can collaborate on. Your free to have as many clients' business' linked to your account as you require.