Staff will be one of your biggest costs if not the biggest and therefore you must carefully control your spend. The good news is unlike the rent and rates which are fixed you can control your staff costs on a daily basis.
There are no hard and fast rules because it depends if you are doing an at seat service, just coffee, coffee and food. What you need to get to is just enough so all staff are busy all the time but not so busy it becomes a sweat shop. You will have busier times during the day, morning and lunch typically. So build your shift patterns around these busy times and DONT be afraid of telling staff they can go home early or come in later.
If you take the attitude of guaranteeing hours to staff in the early days your finances will really suffer. It will take several months to build a good picture of the ebbs and flows of your business so don’t go for a fixed rota until you are sure you have the full picture. For us this took 6-months but we will probably never have a fixed rota because the business needs the flexibility to control the costs.