The amount of cells you NEED will be largely dependent on your cell type: some are more transcriptionally active than others (and indeed, translationally active, since much of the RNA will be ribosomes anyway).

As to the maximum number of cells, that depends on how you're planning to isolate your RNA. If you're using TRIzol, for instance (or other phenol/guanidium isothiocyanate technique), then the answer is "as many cells as you want: just add more TRIzol".

As a benchmark, I would say "a reasonably confluent lawn of cells on a single well from a 12-well plate" should be about as low as you'd go. I can generally get 1-2ug of RNA from that, even in fairly static cell types. Using 6-well plates or larger would be better.

Hi, I don't known what cell lines you used, maybe the attached file can help you.

Your maximum amount of cells needed depends on your application, available cells and and the Extraction methods. In general, there is no maxi amount of cells,  if you need the amount of the material for your down stream application, you will be able to find a way to process.

If you are doing northern blots, you may need a lot more material to start with depending on the specific gene's transcription level. If you run real time qPCR, which may not require a lot of material, a few ug of RNA might be plenty for you.

Trizol and Dynal beads both will allow you to have as many cells as you like to have in one test tube, as long as you expand the volume. But, those column based extract methods such as from Qiagen mini, micro or other RNA kits, every specific kit has been designed to have certain capacity, you will have to follow the specific instructions from the each company.

If you are using Trizol for extraction, the maximum number of cells (or tissue amount) for RNA extraction doesn't really exist. The minimum number instead clearly yes.

If you need at least 1ug of RNA for downstream applications, you could extract 10-15ug or even more ug of RNA and after quantification, take the amount of RNA you need.

Consider that RNA is susceptible to degradation, is better to have more rather than less.

Instead if you're using specific kits, as previously told by Lingli, you have to check manufacturer's datasheet for maximum amount of cells  expected for optimal columns work as they are designed to have a certain capacity.