I found 11 children of John & Jane's. Many of the wills/decrees for this Corkill family are on Brian Lawson's website, so you can see how many children were definitely alive at the parents' death dates.
John at http://www3.telus.net/lawson/twill/1798_001.html
[I see this is under Lezayre in the index - an error].
and
Jane [als Skillicorn] 1823.

Some of the others I transcribed were:
John's father William 1765.
William's first wife Eliz Quay 1738.
His 2nd wife Esther Cannell [mother of John] 1774.
His elder son William, whose decree conveniently distinguishes between his sisters "of the whole blood" and his "brothers of the half blood" 1760.
William senr's probable brother Daniel's will 1762.

Jane als Skillicorn's parents' wills were John Skillicorn 1782, and wife Margaret als Kelley 1789, of Lonan.
Anne Creech als Corteen 1837.

Sue