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The best estimate has been a few thousand. While slaves were prohibited by law from carrying a gun, free blacks had full citizenship (and some even owned slaves) and the south of that time was not very anti-black (ie, whites and blacks worked side by side while in the north, hiring a black man would cause the whites to quit and even slaves were permitted to go to town on their own to buy groceries for their owners, sell things they made, etc) so the free blacks were not in separate black regiments. Why? They were patriotic citizens of their state. This was not a war over slavery although both sides used this tact to convonce some people that war was neccessary. The upstart Republican Party was bent on making laws that would ruin the southern economy such as the huge tariff on English goods so the south left the union (as was legal at the time) but leaving the union would hurt the north badly as most government income came from southern ports and the CSA was a free trade nation so Lincoln declared war on his own and even appropriated funds on his own for the war (both highly unconstitutional).

I'm not sure about Charleston in particular but 10% of southerners owned slaves with 2% having more then five. Like today, in order to compete in an export market, cheap farm labor was needed. The five or less owners generally did the same work as their slaves. Sadly, the south was in the process of ending slavery through the British mode; of slow assimulation. If it weren't for the mass dumping of slaves as free men without preparation, black Americans would not have the problms thay have had and to some extent, still do. 

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