I tried to isolate 1 million cells using RNA isolation KIT and one million with TRIZOL method. Trizol was way better and it gave me approximately 50 µg of total RNA, whereas with Kit it was 10µg of total RNA
you can find several indications concerning RNA extraction from cells, anyway (as you can understand based on the answers already written) the real yield depends on the extraction protocol/kit/technique you adopt.
When I have extracted RNA from HUVEC, I got approximately 10 ug total from a confluent 35mm dish (which should be approximately 1 x10^6 cells) using the Trizol method. Though this was several years ago when I had less experience using Trizol and it will depend on the donor and what you consider "confluent" since I did not do a cell count, so this may be on the low end of yield.
I was considering around 10µg because in Bionumber database the average RNA yield in mammalian cells is 10 µg (+,- 5µg) but HUVEC are always a little bit different. Thanks a lot to everyone.