NSF-situation

QUOTE:
What's the logic of buy quantity or sell quantity?

Re: buy quantity
Please refer to the Help Guide (F1) > Strategy > Strategy Settings > Position Size
Although you can set a Quantity in your strategy code if you need that, size is usually controlled by the selected Position Sizer. Consequently, your code will just PlaceTrade(TransactionType.Buy, ..) and the size is "automatic".

Re: sell quantity
Wealth-Lab is "Position-based". A Position has many properties, like Symbol, PositionType, Bar History object, ... , and Quantity (see QuickRef > Position Class). You strategy code will sell a Position, which already has a Quantity based on the Position.Quantity property, again, usually assigned by Position Sizing logic.