What's supply limit?
For any reason, the supply can never be greater than the total supply limit.
This concept is for Original Hero and Extension only.
Replica has no limit on the total supply limit.
As mentioned in the hero help section, the total supply limit varies by rarity.
Issuing an extension will be done through the crafting. Basically, craft able extensions can be issued up to the maximum number of available for their rarity.
Please note that you can get the extensions no longer issued only by trading with the other users. Also, the supply of each Extension listed in the MCH dictionary is [the number that has been issued] minus [the number that has disappeared due to EMA and the number of crafts in progress].