It sounds like you may have setup a free CRM in place of an actual developer account. Reason being the limit of 10 custom properties is applicable to the free tools. See here for more information.
What I'd recommend doing is navigating to https://developers.hubspot.com/ and clicking "Create a developer account". Once you've done this you should be able to create what are known as "test accounts". These allow you to test apps and integrations without affecting any real HubSpot data. Developer test accounts do not mirror production accounts, but rather have access to a 90-day trial (can be renewed at any stage within the 90 day window) of the Enterprise versions of Marketing, Sales, Service, CMS, and Operations Hub, providing the ability to test most HubSpot tools and APIs.
You can create up to 10 test accounts per developer account. This type of account cannot sync data with other accounts, and they can't be connected to a standard HubSpot account. More information here.
