I am trying to retrieve the scRNA-seq data from a published paper; they say that they performed scRNA-seq on thousands of cells, but looking at the SRA in NCBI, following the bioproject code of the paper, I was able only to find 8 SRA experiments, regarding the raw sequencing. Since I am looking for the expression data, or at least at the raw data for each single cell, do you know where I should look? The link at the bioproject is https://www.ncbi.nlm.nih.gov/bioproject/?term=PRJNA428979
Not every cell gets its own accession number. It is one accession number per scRNA-seq run which then comprises all the hundreds or thousands of cells that were assayed. The accession number stores the raw sequencing data. Most platforms use barcodes to separate the cells (or rather the reads that originate from one cell).
An alternative to downloading data from NCBI is to download as fastq directly from the ENA, see