extent, extant

Extent or extent means the space, measure, length, area, or volume of something, literally or figuratively; how far something goes--
He walked the extent of his land, looking for anything that might need repair or attention.
That's right to a certain extent, but it won't work in this situation.

Extant or extant means still in existence; most commonly used to describe a printed or written work not destroyed or lost--
There are only three copies of this work extant, and they are all in museums.
He mentions the play in his letters, but there is no copy extant.

Now that you know that, you can say--
"To the extent of our knowledge, there is only one copy extant."

Note--"extend" means to continue; to stretch or draw out, in space or time, literally or figuratively--
Does the path extend the length of the beach?
Can we extend the time limit on this project?
"Extends" is the third person songular ( he, she, it ) form of "extend"--
What should I do if he extends his hand? We haven't spoken in years.