There are a few NFkB ELISA kits that can accurately measure NFkB using the fewest number of cells. One such kit is the Cell Signaling Technology NFkB p65 ELISA Kit. This kit can detect NFkB p65 levels in as few as 500 cells, making it a good option for use with 24 well culture plates at a density of 5x10^4 cells/well.
Another option is the Abcam NFkB p65 ELISA Kit. This kit can also detect NFkB p65 levels in as few as 500 cells, and it has the added benefit of being compatible with a wider range of cell types than the Cell Signaling Technology kit.
Finally, the Santa Cruz Biotechnology NFkB p65 ELISA Kit is another good option. This kit can detect NFkB p65 levels in as few as 1000 cells, and it has a high sensitivity and specificity.
Ultimately, the best NFkB ELISA kit for you will depend on your specific needs and requirements. However, all three of the kits mentioned above are good options for accurately measuring NFkB using the fewest number of cells.
Please find attached the comparison for the three kit
CUSABIO Human Nuclear factor-kappa B (NF-κB) ELISA Kit may work in this situation. The kit was designed for the quantitative measurement of Human NFKB1 protein in serum, plasma, tissue homogenates, and cell lysates. It has been cited in 16 articles.
According to your description, you probably use cellular supernatant. However, in consideration of NFkB protein, cell lysate is recommended. But 24 well culture plates are rarely used if the sample type is cell lysate.
No matter what the sample type is, preliminary experiments are recommended.