Hi Niloufar.

6 ml blood samples that were be collected in EDTA tubes will be enough. However, the exact amount depends on your extraction protocol.

From 10mL blood you can expect to get anywhere from atleast 50mn to 100mn PBMCs. That should be more than enough for RNA extraction!

It is usually 1 million PBMCs per 1 mL blood (healthy donors). So from 10 mL blood, you should get 10 million PBMCs. The final yield of course would depend on your PBMC isolation technique. Regardless, this should be enough to do RNA extraction.