If the only consideration is shielding each property from any liability incurred by the other properties, and one individual owns all 12 properties, then it will not make a difference which entity he chooses, as long as he sets up one LLC or Sub S Corp. for each property. Both the LLC and the S Corp afford the safety from liability and both are "pass through" entities, so that the income is only taxed once.
The only real difference would be if there was more than one owner in which case, each member of the LLC is considered to be "self-employed" and each would have to pay a self-employment tax. In an S Corp., only the owner-employee pays a self-employment tax while the remaining shareholders do not pay a self-employment tax.
So, if the properties are owned by only one individual, there is no real difference whichever entity he chooses,
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