You own a store? How long have you been involved in the hobby?
We should be asking you for advice :P Anyways, if you get the clownfish all from the same provider and they are all the same color and breed then you should be able to keep them in the same tank for a period of time. However, you would need a rather large tank. Personally I would split them up in separate tanks. The reason I say this is:
When people buy clown fish they normally buy them in pairs. so if you already have them divided up they will pair up and will make for an easy transfer with less stress for the customer.
I have recently added a second set of clownfish to my 120. One orange pair and one black pair. Luckily they are all easy going and have not had any issues yet. But there is always a risk when mixing clownfish in the same tank.
My local LFS
gets 30-40 clownfish in at once sometimes. He will keep up to 10-15 in the same tank but we are talking 200-300 gallon tanks. He will also keep 6-10 in the same tank as well (smaller tanks). But they are not staying there for very long and they all come from the same providers (for the most part). He also divides up the different types of clowns. Like this time around (last week) he got i think it was 80 clownfish in, all kinds sizes and colors. They are scattered all over the store in different tanks. But he has about 30 in the big tank, and anywhere from 5-20 in different random tanks. But with 5-6 I would not worry too much. Space them out in 3 different tanks, make it easy for you and your customers. I dont reckon these clowns will be around long enough to sex.
Not sure if this helps any, but maybe some others will chime in here and give more advice.