curl --request GET \
  --url https://api.hubapi.com/crm/v3/limits/custom-properties \
  --header 'Authorization: Bearer <token>'

{
  "byObjectType": [
    {
      "limit": 123,
      "objectTypeId": "<string>",
      "percentage": 123,
      "pluralLabel": "<string>",
      "singularLabel": "<string>",
      "usage": 123
    }
  ],
  "overallLimit": 123,
  "overallPercentage": 123,
  "overallUsage": 123
}