How many shops ?

Sure, as long as you produce/buy enough to feed those shops thats fine.
It's important to keep an eye on your shopping value, check it in the manual.

copying from forum.itycoon.net... (apparently the admins have not transfer the posts there back here)

Some players like to have more than one shop of the same type because they could have more flexibilty. Others don't because of the effect the market could have on them if they have only one industry (esp. industries that depend on one or two kind of raw materials only). We can't really answer the "Shall I" part of your question. It is your personal decision whether to branch out or to specialize.

However, you should consider the following factors before you make the decision:
availability of raw material / products (self-supply -> map vs. buying from others -> market)
SuDe (supply vs. demand for the shop... and the resulting drop of price)
personal goals (gain experience in various industries vs. staying safe)
strategy (research? expansion? profit vs. highscore, or both?)

It is your choice and there is no "right" or "wrong" as long as you can justify it with reasons.

btw... "Shall I" is different than "Could I", which both firma and wwf answered to