For best results, with community targeting, it is recommended to start with:

The correct answer is "At least 15 communities per ad group, and/or combine Community & Interest targeting together in an ad group." This approach is recommended for Reddit Ads because it provides sufficient audience volume for the algorithm to optimize delivery effectively. A broader selection of communities, or combining community targeting with interest targeting, helps ensure your ads reach a substantial and relevant audience, preventing under-delivery due to overly narrow targeting. Starting with "Communities with the highest number of subscribers" isn't always optimal as subscriber count doesn't guarantee relevance. "A single, highly relevant community" is too restrictive and will likely lead to insufficient reach and poor performance. "As many communities as possible to maximize reach" is too vague and could lead to irrelevant targeting, diluting your ad spend. The recommended minimum of 15 communities or combining with interest targeting strikes a balance between relevance and reach.