No, the 5 at a time is referring to the number of vehicles used for the business at the same time. Per tax laws, the standard mileage deduction cannot be used if 5 or more vehicles are being used by a business at the same time.
See more information here from this IRS Topic 510: Business Use of a Car:
Standard Mileage Rate - For the current standard mileage rate, refer to Publication 463, Travel, Entertainment, Gift, and Car Expenses or search standard mileage rates on IRS.gov. To use the standard mileage rate, you must own or lease the car and:
You must not operate five or more cars at the same time, as in a fleet operation,
- You must not have claimed a depreciation deduction for the car using any method other than straight-line,
- You must not have claimed a Section 179 deduction on the car,
- You must not have claimed the special depreciation allowance on the car, and
- You must not have claimed actual expenses after 1997 for a car you lease.
To use the standard mileage rate for a car you own, you must choose to use it in the first year the car is available for use in your business. Then, in later years, you can choose to use the standard mileage rate or actual expenses.
For a car you lease, you must use the standard mileage rate method for the entire lease period (including renewals) if you choose the standard mileage rate.
https://www.irs.gov/taxtopics/tc510
