Honestly, when it comes to futures, the more $ you have, the better it is. Accounts under $25K are pretty limited in terms of the strategies you can deploy & how you can manage & adjust your current positions.
I'm glad I didn't start with more than 8k when I did, because I would have likely thown away 8m if I had it.
The answer to this depends on skill. I am day trading crude for the past few months with a 5k account and still have all my money. Took seven years to get to that point where I won't lose it all in a week.