There are two ways of interpreting it, one in the spirit of the binary operation asking "are these mutually exclusive and exhaustive options", in which case the sentence is
... is true when one, and only one, of the n sentences is true.
as Christopher says.
The other is like the result of putting this into a computer. As XOR is both commutative and associative, we can put any statement of the form
A1 xor A2 xor ... xor An
into a cannonical form, putting all the trues first and all the falses last
T xor T xor F xor T xor F
could be written
(((T xor T) xor T) xor F) xor F
so it is only the number of trues and falses that matters. XOR can be identified with addition mod 2, with false being 0 and true being 1 . So it is the number of ones (trues) makes a difference to the final result, i.e.
X xor F -> X (nothing changes)
X xor T -> not X (value flips)
so, the truth is determined by whether there is an odd or even number of "trues". Then we have
T xor T is false
so even numbers of trues give false, and the sentence you would be looking for is
... is true when the number of true sentences is odd?