By default AmiBroker backtesting engine, sets some Initial capital and displays ending capital in backtest result, but more often we need to backtest a strategy with a given number quantity instead of how much trading capital we have, as later criteria can pose problems with options where If one has fixed the trading capital, then number of traded quantity might vary over trades.
To do this we only need to add one line towards the start of AFL file
SetPositionSize( 1, spsShares );
Here 1 being quantity, also if you are backtesting multiple symbols it is preferred to set max number of open position to 1000, this can be done from
Analysis window -> backtester settings -> portfolio tab