Is each client tracking their own data and you have oversight of it from your computer? You could do that with a Google spreadsheet. Be the “owner” of the spreadsheet and invite each client to the spreadsheet. The only bad thing about that is that clients can see others’ data.
The other way to do it with a google spreadsheet would be to write a questionnaire that feeds into the google spreadsheet. The answer for each question goes into a column on the spreadsheet. With that, individuals would only see the questionnaire and provide their answers, and you’d have comparative data on the spreadsheet.
Just make sure that the first few questions of your first survey include the client’s name and other critical data you need to identify them. Otherwise you’ll have a big spreadsheet with data but not know whose is whose. 😀