Getting a forum to really "catch fire" is always the hardest part. Once it's going, it will be self-sustaining (but will still require moderation).
The question, "how many members in a year is a good number" is one that's really impossible to answer. A niche forum on, say, Civil War autographs might never grow beyond a few hundred members. Others add that many daily.
I'd look at other measures of success:
- Is conversation self-sustaining, or do you have to keep starting topics, bumping old ones, etc., to avoid a dead-looking forum?
- Are posts, thread starts, and daily new members trending up, down, or flat?
Generally, if your numbers are trending up and you can pretty much stand back and moderate, the community has hit critical mass. Faster growth can be achieved by outside promotion and adding viral features.
As far as adding content posts yourself, I wouldn't obsess about the SEO but I would be sure these pages have appropriate titles.