How do I count the line count of a script?

You would have to get the source of the script(as Spongocardo stated) and reconstruct the entire text-I had already done something like this. This is heavily based off of BuildIn Libraries' Helper Library.

function getSource(s)
    local lines = {};
    local str = "";
    for i = 1, string.len(s.Source) do
        if (string.sub(s.Source, i, i) == "\n") then
            lines[#lines+1] = str;
            str = "";
        else
            str = str..string.sub(s.Source, i, i);
        end
    end

    if (str ~= "") then
        lines[#lines+1] = str;
    end
    return lines;
end

Now you could do #lines to find how many lines it has.