This has to do with your Clarizen Edition (i.e. Professional, Enterprise or Unlimited). Depending on the edition, your customization limitations will differ. This can be viewed in your Settings-->Configure and from the left Item Type panel, select Quota.
Also, limiting simultaneous calls is required to avoid performance issues. If you want to find out how to work around this. Please post your question with your Use Case (in detail) and be sure to post it to the Customization Topic.