A CFD broker will let you open a trade on margin as long as your account balance is more than the margin required on all your open trades. If the required margin increases within a certain percentage of your account balance, you will get a margin call. If you then don't deposit more funds or close losing trades out, the broker will close all your trades.
Note: Your account balance is the remaining funds you have left to open new trades with.
I always use stop loss orders with all my open trades, and because of this my broker reduces the amount of margin required on each trade. This allows me to have more open trades at the one time without increasing my funds.
Effects of a Losing Trade on Margin
Say I have an account balance of $2,000 and open a long trade in a share CFD of 1,000 CFDs with a share price of $10 and margin of 10%. The face value of the shares would be $10,000, but my initial margin would be $1,000 (10% of $10,000).
If I don't place a stop loss and the price falls to $9, I would have lost $1,000 and my remaining margin would now be $900 (10% of $9,000). So I would have $100 balance remaining in my account. I would probably receive a margin call to deposit more funds in or close out my trade. If I don't respond the broker will close out my position before my balance gets to $0.
If instead I placed a stop loss at say $9.50, my initial margin might be reduced to $500. As the price drops to $9.60 I would have lost $400 and my remaining margin would now only be $100, with my account balance at $1,500. When the price drops to $9.50 I will get stopped out, my trade will be closed and I would have lost $500, with my account balance still at $1,500.
Effects of a Winning Trade on Margin
Say I have the same account balance as before and open the same trade but this time the price moves up.
If I don't place a stop loss and the price goes up to $11, I would have made a $1,000 profit and my remaining margin will now be $1,100 (10% of $11,000). So my account balance would now be $2,000 + $1,000 - $1,100 = $1,900.
If I had placed a stop loss at say $9.50 again and the price moves up to $10.50, I would have made a profit of $500 and my margin would now be $1,000. My account balance would be $2,000 + $500 - $1,000 = $1,500.
However, if after the price went up to $10.50 I also moved my stop loss up to $10, then I would have $500 profit and only $500 margin. So my balance in this case would be $2,000 + $500 - $500 = $2,000.
So by using stop losses as part of your risk management you can reduce the margin used from your balance which will allow you to open more trades without any extra funds deposited into your account.