Ask them now. You should be signed up for lsdas with lsac, letters will go to lsac. 1 letter per reccomender unless there is a compelling reason to ask someone for a school specific letter.
Ask ASAP because some profs will be slow in writing them. Just ask for one generic letter that they will mail/fax to LSDAS and it will be sent to all the schools.
awkward follows you like a beer chasing a shot of tequila.