I don't have much experience in this area, but it makes sense to use a constant cell concentration for each experiment. This is essential to compare and analyse the results. If you use a comercial ELISA kit for your tests, you can try several cell concentration and in respect with kit instructions decide which one is the best. If it is a inhouse ELISA you also have to test different concentrations using positive and negative known controls. The implementation of a new test request prior work that establishes the ideal settings for repeatable and reliable results. The cell concentration to be used may differ from a cell line to another.
Cell number is very important in ELISA, because it effects the growth of cells. If cells are over confluent you may get erroneous results. Generally, adherent cells are seeded in lower numbers as compared to suspension counterparts.If you are using adherent cell line i will suggest you first seed (in a flat bottom 96 well plate) the cells in 8 x 104 , 4 x 104, 2 x 104 ...... and observe after 12 hr for confluency. chose the cell no were confluency is 70- 80 %. Hope, You will get good and constant results. Also it is better to use Colorimetric caspase activity assay system...