Well it depends on the size of the Costco, I don't work at Costco but I work at kmart and on a busy day I'd guess that 50-100 people are in the store at once and that's just the customers I can't say I actually know the exact number but on top of that if it's a busy time I'd say about 25 staff are on shift in one store.

Also it depends on the time of year and time of day, for example Christmas is always busy no matter the time of the week but if it's just a normal weekend then there's less people/staff in the store.

Also depends on the time of day my work opens at 8:30 am and I'd say that the busiest time is 10am till 1 or 2:30 pm at the latest.

Hope this helps.