Devote about sixty percent of your rack space to cards that are in demand year round like birthday, anniversary, thank you etc. The remaining 40 percent should be filled with date-sensitive product like Valentine's Day, St Patrick's Day, Mother's Day, Father's Day etc. that are only displayed when in season.
The wholesalers you buy from will be a lot of help in advising what sells best in your area. For an preliminary estimate, a 5 tier rack, stocked three cards deep would require 30 cards per lineal foot allowing six inches width for each card. Done that way, you can estimate the number of cards from the amount of floor space allotted for cards. For example a 20 foot display would hold 600 cards and a 10 foot display, 300.
