I think focusing on 10-20 keywords is better beacuse you can arrange gettin better serach engine quarries for less words.
it's always better to start with small quantity of keywords, check the results and, if profitable, add new keywords. And don't forget to play with exact match, phrase match and broad. It makes huge difference for your CTR and bids!
For the content network 50 per adgroup max. For the search network 25 per adgroup max. As for totals, 2,000 for content seems to get a lot of traffic fast, but for search , I'd test one or two adgroups at a time.