If there are no soul counters on Obscura Ascendancy, its ability triggers if you cast a spell with mana value 1. If there is one counter, its ability triggers if you cast a spell with mana value 2, and so on.

Obscura Ascendancy's triggered ability has an “intervening ‘if'” clause. This means that it checks the number of counters both when the ability triggers and when it resolves. Notably, this means that casting two instants in a row with mana value 1 will never cause a player to create two tokens or put two counters on Obscura Ascendancy.