I have at over 10 lol, a good money management system is the one where you bet less than 2% of your bankroll on bet so you can recover even in very bad series of loss.
Ofcourse you decide how much % you bet, but dont go fot the big wins.

Its good to have multiple accounts because most sites offer a first time deposit bonus so you can keep getting free money. Just make sure the sites are trustworthy and accept americans.

Absolutely. If your serious about sports betting (investing), then I would highly encourage you to have at least 4-5 funded accounts at quality sportsbooks. For a little more information on what I look for in a quality sportsbook, here are my personal criterion http://www.sports-betting-insights.com/choosing-a-sportsbook.html . For the sportsbooks I personally use, which are extremely quality and highly rated, check out my favorites here http://www.sports-betting-insights.com/best-online-sports-betting-sites.html .

As for money management, this is by far the most important factor when it comes to betting on sports. This is the one criterion that you have control over and you must be very disciplined. I could go on and on and on about money management and how important it is but it's probably just better to direct you here http://www.sports-betting-insights.com/money-management.html .

Best of luck.