Nextcoin seems to convert 256-bit Curve25519 256-bit public keys into "account numbers" that have only arabic numbers and seem to only have a length of 20.
How is that done?
In the Nextcoin forum, users leave their account numbers.
According to these lines an accountId seems to be...
...which translates to 256^8 accounts or roughly 2 * 10^19, which fits the idea of around 20 arabic numerals.
Given there are only 2^64 unique accountId and the birthday paradox, you can check this collision probability table (the 16 bytes/64 bits row). If each person in the world had a single Nextcoin account, the collision probability is greater than 75%! I'm sure this is the least of their problems.
This looks really amateurish. Really amateurish. Really, REALLY, REALLY amateurish. Stay away from this coin!