Where a bill of exchange is drawn in a set of six, how many of them need to be proved?

The question relates to Explanation 2 of Section 91 of the Evidence Act. Section 91 reads as - Evidence of terms of contracts, grants and other dispositions of property reduced to form of document. –– When the terms of a contract, or of a grant, or of any other disposition of property, have been reduced to the form of a document, and in all cases in which any matter is required by law to be reduced to the form of a document, no evidence shall be given in proof of the terms of such contract, grant or other disposition of property, or of such matter, except the document itself, or secondary evidence of its contents in cases in which secondary evidence is admissible under the provisions hereinbefore contained. Explanation 1–This section applies equally to cases in which the contracts, grants or dispositions of property referred to are contained in one document, and to cases in which they are contained in more documents than one. Explanation 2– Where there are more originals than one, one original only need be proved . Explanation 3. –– The statement, in any document whatever, of a fact other than the facts referred to in this section, shall not preclude the admission of oral evidence as to the same fact. Illustration c - If a bill of exchange is drawn in a set of three, one only need be proved.