The present study investigates the relationship of the year-to-year variations in the tropical cyclone (TC) genesis number over the western North Pacific (WNP) to the intensity of 10-20-day and 30-60-day intraseasonal oscillations (ISOs) during 1979-2018. It is found that the relationship between the TC genesis number and ISO intensity differs largely among the four quadrants of the WNP. The TC number in the southeast quadrant of the WNP has a close relation to the ISO intensity, but the relation is mostly weak for the TC number in the other three quadrants of the WNP. More (less) TC number in the southeast quadrant of the WNP corresponds to a stronger (weaker) ISOs. This indicates that stronger ISOs or longer active phases of the ISOs provide a favorable condition for formation of more TCs. This modulation of the ISO intensity on the TC genesis number suggests an indirect way for influence of seasonal mean background on the year-to-year variations in the WNP TC number through the ISO intensity. Further analysis suggests that this indirect way is likely a more important manner for the influence of seasonal mean background on the year-to-year variations in the genesis TC number over the WNP.