What does the number mean in an HGVSp annotation? Let's take the example p.Arg452Pro that I got from an annotated VCF file that contains an HGVSp column.

What does 452 mean in this case ? 

Theoretically, is it possible to have the same HGVSp on the same gene but on a different transcript level ?

For example:

Chromosome      Position        Ensembl_Gene        Ensembl_transcript      HGVSp
4               4613173         ENSG00052342        ENST00141456            p.Arg452Pro
4               4613176         ENSG00052342        ENST00141458            p.Arg452Pro


Thank you in advance.
It's the position in the protein. p.Arg452Pro means that there is a variant that changes an Arginine residue at position 452 of the protein into a Proline. 

And yes, it is absolutely possible to have the same HGVS p. notation on the same gene but on different transcripts. More than possible, it is quite common. Your particular example isn't actually a real variant since that position on chromosome 4 is intronic in both hg19 and hg38, but TP53:p.R175L1 is a real example of what you describe:



As you can see in the image or by visiting the link, this variant on TP53 has the same HGVS p. notation on 6 transcripts. It also has a different p. notation on two more transcripts that are not shown in the image as well as no p. notation for a few other transcripts in which it doesn't affect the protein product.



1Disclaimer: The link goes to https://varsome.com, a free human variant search engine that is developed by the company I work for.