What is the maximum quantity of issuance?

Countewallet allows over 92 billion (92,233,000,000).
In the API and CLI, up to 2^63-1 is accepted (e.g. 9223372036854775807; which is 2^63-1.

Note that owners of un-locked assets (tokens) can issue additional amounts, but at the moment the amount cannot be decreased so sometimes it is better to start with a smaller issuance and avoid the need to unnecessarily huge amounts.
Other times (especially for locked assets) it is better to get the quantity as “right” as possible.