Should I consider one or two DNA chains to find how many nucleotides are in a gene?
Counting both strands is admirably correct, but most people will multiply the number of amino acids by three, not six.  For example, nucleotide positions in a reference sequence will increase by 3 for each amino acid in the presumptive coding sequence.  Besides, if you want to be that pedantic, you should say zero, since the DNA contains nucleotide residues, not individual nucleotides, as they have undergone dehydration synthesis.  I think in biology the right answer will almost never be marked correct by a machine. :)
The answer you come up with will also be influenced by whether you would like to include a stop codon (+3) to avoid nonsense-mediated degradation, and a transcriptional start site and polyadenylation sequence (in eukaryotes).  The ''gene'' itself, in the genetic sense, will include enhancer elements over a much longer region of the chromosome and regulatory 3'-untranslated regions.  And then there's alternative splicing to consider...