The 300GB is for "all public sequences", as explained. But to answer your question about "one virus", the initial isolate of SARS-CoV-2 from Wuhan, China has a

The 300GB is for "all public sequences", as explained. But to answer your question about "one virus", the initial isolate of SARS-CoV-2 from Wuhan, China has a genome containing 29,903 nucleotides. You can see the full raw sequence by clicking on "FASTA" on this page: https://www.ncbi.nlm.nih.gov/nuccore/1798174254 https://www.ncbi.nlm.nih.gov/nuccore/1798174254