How many employees would be allowed off at any one time in a dept with 8 full time staff?

you just have to make sure all major functions of the business have someone present who can do it all times - payroll, check writing/signing for bills, invoicing, collections, order taking/customer service etc - I would think any more than 2 people out at one time for a full week - vacations, etc, would be too many

As a rule in general for that number of staff the logical answer would be two, but it depends on the company. This should be laid out in your contract of employment or as agreed on your employment terms.

That would be a matter of internal company policy.