The description says, we can earn it by:
Provided non-wiki answers of 15 total score in 20 of top 40 tags
Does it mean a 15 score in each of the tags, or in cumulation?
It's definitely not 15 total score across all tags.
According to the List of all badges with full descriptions, it requires a cumulative answer score (all upvotes minus all downvotes) > 15 per tag in 20 of the top 40 tags:
- silver; awarded once
- Earn a tag score of 15 on answers belonging to one of the top 40 tags
- Repeat the above step until any 20 of the top 40 tags are covered
Also! Be aware that the Generalist badge requires the site to have 200 questions in each of the top 40 tags before anyone on the site can get the badge (from http://blog.stackoverflow.com/2010/05/generalist-badge-implemented/):
One thing the badge doesn’t say, is that there must be at least 200 questions in all 40 of the top tags before this badge is awarded to anyone. ... I don’t feel you can accurately measure a generalist until the top tag list settles down.