Speed Card Numbers, think of them as a level. Higher the level is typically better, as it increases your overall car level. Thus, giving you higher drops in races and the tune shop will restock with higher cards.
Stats wise, think of what you want for your car. Do you want acceleration? Speed? Braking Power? Then aim for the bonus stats on the card, or if you hit the "Help" button in the tune shop it will give you some hints. It will also tell you what brands give what bonuses. Equipping a minimum of 3 cards with the same brand give a bonus to certain stats. If you equip 6, it will give you another bonus. Or you could chose the route of going with two different brands, so you have two different "x3 Bonuses".
Hopefully that helps a bit, if you have any other questions feel free to ask!
