I am going to apply for a multiple-entry visa to Netherlands this coming May. If I will be given only a single entry how soon can I apply again for another Schengen visa because I am planning to go to Paris in July too?
Alternatively, if you can present solid documentation for both the Netherlands and Paris trips, you can try for a multiple-entry visa. These are usually not granted if it's your first visit to Schengen though.
You could apply immediately after coming back and go back to the Schengen area as soon as you get the visa, there is really no restriction on that. If there is enough time, you could also probably apply before your second trip (consulates usually avoid issuing overlapping Schengen visas but as long as your first visa expires before July, there is nothing stopping another consulate to issue another visa for the second trip, even if you haven't used the first one yet).
But whatever you do, whether you get several visas in short succession or a long validity multiple-entry visa, you cannot stay more than 90 days during any 180-day period on short-stay visas. So if your first stay is too long, you will not be able to get a new visa, making other restrictions unnecessary.