The client only has one log-in to view the client portal, even if they have been invited to multiple assignments. When they sign in to the portal they can switch easily between the available assignments by clicking the 3 dots icon in the top right corner (1), then click the Projects icon (2), and select the project they want to view (3).
This approach will work even if they have been invited to view a client portal by another firm, in addition to yours.