Should it be "fashion" or "fashions"?

As you say you are using a brand name and I presume 123fashion(s) is the brand name, your usage of s or not will depend on how the brand name is spelt. If the brand name is 123fashions, then use it always with the s, if it is 123fashion, then use it without the s. This is because brand names are sacrosanct to brand owners and brand managers and they will not accept any truncation or addition to what they call their "name and style"

Fashion Or Fashions

I do not know what type of product you are going to sell, but "123 Fashions" (it WOULD have an "s" in this context, unless you are saying '1...2...3.....FASHION!!!!' like 'ready...set....go! type of thing) really does sound cheap, like you are rushing a product out there (the "123" part).
I work in the fashion industry, and rarely(like never) do you see "fashions" in a name at all. But, again, I haven't seen your product.
Good Luck!

Gramatically, I don't know but 123Fashion sounds better cause it's a brand name.

It depends on the context it is used in. It can be either fashion or fashions. In the context you have stated above, I would say it would be: "123 fashion" , as it is a brand name.