The short name quota applies to every time a user changes the short name of a link that has previously been changed (not including changing from a generated name to a custom name). The quota will be available again after the date of resetting the quota or upgrading to a higher service.
s.id/Aks12 (automatic link created by the system) you customize to = s.id/custom (This does not eat up quota)
s.id/custom (after the first time you custom) you customize it to = s.id/custom1 (this just eats up quota)
free : 5 quota/mo
Lite : 10 quota/mo
Starter : 30 quota/mo
If you further question, contact firstname.lastname@example.org