sorry for the naive question, but how do I determine what the copy number is in GRCh37 for a gene with multiple CNVs? (e.g. DRD4).
A very naive approach would be to simply ask how many copies of the sequence in question are in the hg38 sequence, at some level of homology. You could do this using a tool such as BLASTN (which has a web interface that you can use to search against a human genome) or, if you have the sequences downloaded somewhere where you can work with them, using a tool such as minimap2 (which will be faster) as long as you're comfortable using command-line tools.
If you choose to do this, you will have to be comfortable setting thresholds for what counts as a copy of the gene in question.
A more involved solution would be to obtain DNA sequencing reads for the sample you're interested in and use them to quantify the copy number of the gene based on the number that map to a given sequence of interest. An older approach would be to use microarrays.
I am guessing that since you are interested in the reference genome that you are not interested in going to that kind of trouble.
Is this a homework question? If so, you should add the "homework" tag.