Short answer:
Figure out how many you need and from whom (if specified) Make sure if a school requires more LORs than just the one's sent to PharmCAS you don't have the same people write the secondary LORs (e.g. Western Univ). Third, especially if you have the same people writing multiple LORs, give them a lot of notice (Before fall quarter/semester starts if at all possible because once school starts, JOB is 1st priority, LOR is 2nd)
Long Answer:
I would say, get a feeling for which schools you plan to apply to. Figure out which use PharmCAS and which don't. For the ones that use PharmCAS, see what their LOR requirements are. PharmCAS doesn't stipulate who can and can't write LORs. For all they care you could have your parents and a brother write them. Some schools require a pharmacist, others don't allow employer LORs. Look at the schools that don't use PharmCAS for the same. Just remember, the ones that don't use PharmCAS means that you'll have someone writing multiple LORs for you or you'll need multiple people to cover all the LORs. For me, I don't know any pharmacists and didn't want to go to a pharmacist to have them interview me and write one based on the interview or try to get into a pharmacy without my Pharm Tech license. That meant any school requiring an LOR from a pharmacist meant I was SOL. (I don't suggest following my lead here, definitely at all costs get pharm experience or at least acquainted with a pharmacist). Western U requires LORs in addition to PharmCAS. In all I had 6 professors and my supervisor at work professors write 9 LORs. My Medicinal Chem professor I had write two and my Pharmacology teacher wrote two. It was hard on me because I had to think of who knew me well enough to write an LOR. Since the courses I had taken with them was on the LLU LORs I also had to consider which classes would it be best for them to see LORs coming from. (Med Chem and Pharmacology LORs I figured looked best from a Pharm School standpoint)
I don't know which is worse all the work it took to get the LORs out, follow-up, and get them sent back, or getting the nerve to ask my profs for another LOR when they'd already sent one in for me. (You'd have to know I hate relying on anyone for anything so to ask once was hard, but I knew it was necessary. To come back and ask for another one I really hated.)