curl --request GET \
  --url https://api.hubapi.com/crm/associations/v4/definitions/configurations/{fromObjectType}/{toObjectType} \
  --header 'Authorization: Bearer <token>'

{
  "results": [
    {
      "category": "HUBSPOT_DEFINED",
      "typeId": 123,
      "label": "<string>",
      "userEnforcedMaxToObjectIds": 123
    }
  ]
}