It depends on what kind of spell you're doing. There's love spells where the love will be burned in the persons heart when the flame touches the pins, or that when the pins fall the person will fall in love with you. Also I've read about it being used in commanding spells when the pins fall it means that the persons will is following your command.
I'm sure it's being used in other ways too, those are the ones that I remember now. And do read up on instructions before doing any spells so that you are sure of its intention and purpose.
Depending on practice and beliefs. As well as the pin itself along with the candle. As said above me using pins in a candle can be used for 'love' spells or of that sort. They can also be used to bind your will and intent.
When I put pins into a candle. I am insuring the reason and cause for my desired or want is worth whatever is return. It is my way of speaking whatever comes is worth what I am putting for this. In this I do not do lightly. If need is for a friend or client. I use the pin by santisizing it and pricking them. And stick the pin in. Binding them. Before I do such an action I clarify why and that they accept the effects. The pin in my mind is signing a contract.
The number should be on what numbers mean to you. Perhaps look into numerology for a number that briongs meaning to you. Perhaps two symbolizing you and another. Another thing to consider is the height of the candle.