To get the number of entries in a variable, you can use the len function. Something like {{ $data | len }}.
I’m not quite sure what you’re asking about with the replacing strings thing… if signees.json has something like
{
"foo": "bar baz qux"
}
and you want to replace “baz”, you could use replace (or replaceRE if you need something a little more flexible). Something like {{ replace $data.foo " baz " $whatever }}.