What is an efficient way to spike 10-50 cells to a culture
If you are you working with primary cell cultures or cell lines may be you could do this when cells are in suspension, after tripsinization, mix them well with a 5mL serologial pipete in a falcon 50mL tube and plate to a 100mm2 with a final volume of 7mL. And finaly draw an eight with the plate in the hood more than 3 times to mix them well.

Edit: For the addition of 10-50 cells, count these cells with trypan blue, and do a 1:100 dilution, for example if you have 1e6 cells in 10mL you could take 0.01 mL (10 microliters) to get 1, 000 cells 





And then you add the 0.01mL into 1mL of cell media to get 1 cell per 1 microliter, and then if you wanna get 10 cells, take 10 microliters, and so on