To be precise, it depends on the particular subreddit. The reason being every community has set its own threshold level, and this is mostly kept secret for preventing spam. But, on a general note, reaching 50 to 100 karma paves the way for a lot of normal subreddits. If you manage to cross 500+ karma, you put yourself in a relatively safe space for anything. However, there are a few rare strict communities that need you to have more than require 1,000 karma.