Your question is too open and wide of scope to answer easily.
Are you referring to direct customers of Cisco Meraki who own the actual Cisco Meraki devices, or customers of a business that use Cisco Meraki?
Obviously data that you type into the dashboard is stored there, such as network names, organisation name, device names, device locations, organisation and network admin email addresses, etc.
If you use Meraki splash page authentication for the customers of the business that owns the Cisco Meraki kit (such as a cafe) with Meraki hosted authentication then their email address (which is their login details) are stored.
Perhaps you should take a look at the Meraki Trust section of the web site to see if it can answer more specific questions.
https://meraki.cisco.com/trust