How many fire extinguishers should be in a 5000 square foot building?

It depends on the extinguisher rating. Let's say its rating is 13A.

Multiply the floor area (5000) by .065 and divide that by the rating (13, here).

5000*.065=325

325/13=25 extinguishers

If they are 21A extinguishers, then:

5000*.065=325

325/21=15.48 -> 16 extinguishers (always round up)

If it's just a math question ignore the rest.

If it's a serious question, and not just a mathematical one, then it should be noted that if there are multiple floors those numbers could change. Also, normal fire extinguishers are (some number)A (these are for solid flammables such as wood, paper, textiles......). if they are B instead (the tag on the extinguisher will say and these are for liquids) then the calculation is different.

Go to any fire department and ask a firefighter. They're sure to know the answer (or will at least have quick, easy access to the answers).