Can my client take multiple questionnaires?
Yes, your client users are able to access multiple questionnaires. To get your client into their first questionnaire, follow the steps in the article linked here: How to invite a client to a matter
There are 2 ways to give a client access to another questionnaire/matter:
Option 1. Create the new matter from the Client Users screen
From the left hand navigation go to "Client Users", then search & select the client user:

On the next screen you can click the button to add another matter/questionnaire for this client user:

This opens the following dialog. Here you can choose whether the system should email the client about this, and pick the name and questionnaire type:

Option 2. Create the matter first, then invite the existing client
The other way to let a client user work on an additional matter is to create the matter first, and then invite the existing client user to it.

Then type a Matter Name and choose the type of questionnaire.

Open the matter - and navigate to the "client users" tab. Select "Manage Client Users":

Then from the "Existing Client User" tab search for the client user. Here as well the tool will show a dialog where you can pick whether it will send a notification to the user:

Frequently Asked Questions
Does my client need another account for the second questionnaire?
No. You attach the existing client user to the new matter, and they use the same login for all of their questionnaires.
Will my client get an email when I add the new matter?
Only if you want them to. Both options show a dialog where you choose whether the system sends a notification email to the client user.
Which option should I use?
Option 1 is fastest when you are starting from the client: everything happens from their Client Users record. Option 2 fits when the matter already exists or you prefer to set it up first and add the client after.