How many chlorine tablets should I buy at one time?

Depends on your normal usage, and what storage you have available.

You don't normally want to buy more than you can use in a year. And, if you have to store the tablets indoors in a damp or humid climate, you probably want even less. Once they get a bit damp, the tablets emit fumes that can seriously damage wiring in a utility building or garage. (Do NOT bring them into your house.)

You CAN buy something like a 50 gallon Rubbermaid BRUTE container, and store your tablets OUTSIDE, but inside the container. If the address you reported in your registration is correct, then you could put the tablets in a BRUTE container on your pool deck, but inside the screened area.

However, before you get a large container of trichlor, you should probably test your pool's stabilizer level ACCURATELY -- strips used by you or a dealer don't count, especially for CYA testing. If your stabilizer is already high, you probably don't want to purchase a bunch of trichlor, which will add even more stabilizer. If you purchase the test kit from the Amazon links in my signature, you'll be able to test it yourself accurately.