How only one follicle develops into graffian follicle?
It doesn't mean only one follicle maturates within a lifetime of a woman. For every ovulation only one follicle maturates from the batch of primary oocytes stored within the ovary. Before a female is born, all the primary follicles are arrested in the diplotene stage of prophase I. Still, we do not know exactly the mechanisms that let only one follicle to release a mature egg cell, but studies have shown why it possibly happens.



Granulosa cells surrounding the oocyte have receptors for FSH (follicle-stimulating hormone), FSH activates the production of aromatase, which transforms testosterone produced under the influence of LH (luteinizing hormone) in cells of theca interna into estrogens. Estrogens activate expression of LH receptors on the surface of granulosa cells and further stimulate the production of LH. The more LH activates thecal cells, the more estrogens are produced in granulosa cells and that at the same time increases production of LH receptors on granulosa cells. 

FSH also stimulates granulosa cells to proliferate, providing the whole follicle with a large number of FSH receptors and increased production of aromatase and at the same time stimulates the follicle to finish meiosis I. Eventually one of the growing follicles will have enough FSH and LH receptors on the granulosa cells to become able to respond to large surge of LH, which results in ovulation. The rest of the follicles that started to develop degenerate due to insufficient number of FSH and LH receptors. The fact that FSH and LH concentrations drop rapidly after ovulation also halts the development of remaining follicles.

The follicles that did not receive enough FSH, won't grow to the point, where granulosa cells start expressing LH receptors and they won't pass through the 1st meiotic block, therefore they still may be activated in the next month. Those that did pass the 1st meiotic block but couldn't be ovulated, will degenerate.

It is still unclear whether there are some additional mechanisms that help the atrophy of underdeveloped oocytes.

Sources:


Larsen's Human Embryology
https://kjcscientific.wordpress.com/project-excalibur/