During my visit to Holland, we had the opportunity to visit Keukenhof as I love tulips. Keukenhof is open every year from March to May and it's situated in Lisse. According to the official website for the Keukenhof Park (http://www.keukenhof.nl/), approximately 7,000,000 (seven million) flower bulbs are planted annually in the park, which covers an area of 32 hectares.
You can buy the ticket online and I would advise you to buy the combo ticket (bus & entrance ticket). Just decide from where you would like to board the bus (from Amsterdam/Schipol/Harlem/Leiden). An adult ticket will cost you €15 and a combo ticket will cost around €22-27.
Unfortunately, due to the long winter, some of the tulips hasn't bloomed yet. Nevertheless, I managed to take picture of some of the tulips and also a few lavenders. Just look at how beautiful they are.....