For every market that you want to trade, a good rule of thumb is to triple the margin and put that into your account. With that much money you should be able to trade that market using the Kenison method without ever running down to zero (although please read carefully this disclaimer). Remember that there is serious risk of loss trading futures. It is not for everybody. You could lose more than your original investment.
You do not have to trade every market. You can trade just the ones you want, or just one type of market (such as grains only), or, you can pick one from each type (e.g. one metal, one grain, one currency) and so get some balance in your account.
As your account grows you can always add to the number of markets you trade.