sativa and indica dont have different amounts of leaves, its all about the plants age, say the plant is a seedling it will only have one leaf, say its a young plant it will have 3, then 5, then 7, sometimes you get more than 7 but thats due to deformaties and mutations.
there are a few ways to tell the difference between indica and sativa, sativa is thin, with sharp serrated points along the edge of the leaf, whereas indica has round, stubby leaves with less pointy edges, also another rough indication is that sativa plants are a brighter green than the dark indica.
to find a 'completely pure' sativa plant or indica plant has become increasingly difficult over the years, although some seed sellers still produce the pure plant people often cross-pollinate them, (creating a new plant) this can increase yeild and decrease growing time therefore making a better, easier to grow plant.
this is all from memory so its not 100% but i hope it helps