X=5 was suggested on Slack by @oxinabox.

If anyone thinks we should not have such a check or would like to bikeshed the value of X, please comment here! (Would rather get that out of the way before implementing it...)

Lyndon also suggested a nice error message:

Error message can be something that's says:

• you seem to have registered a near empty package. Did you forget to push your commits?
• Name squatting (E.g. for a package you plan to write later) is no permitted.
  (Also need to add this to the readme)
  please reregregister once the package contains usable functionality.
  If we have detected this incorrectly (e.g. you package is short but useful) please comment for manual merge.