One of the largest hurdles for genome-wide CRISPR screening is ensuring adequate coverage of the sgRNA library across the cell population. Evidence so far has suggested that each sgRNA should be represented and maintained in a minimum of 200-300 cells.
Considering that the standard protocol uses a multiplicity of infection of ~0.3, and a transduction efficiency of 30-40% the number of cells required to produce and maintain suitable coverage becomes very large. By way of example, the most popular human sgRNA library is the [https://www.addgene.org/pooled-library/zhang-human-gecko-v2/?gclid=Cj0KCQiA1-3yBRCmARIsAN7B4H2cTsJq44UZen4GOpdr-IbSGuTdqS_BjcCdi9x0PtthkDXvoIztPDgaAoNIEALw_wcB/ GeCKO v2 library] created by the Zhang lab; it contains 123,411 sgRNAs. Studies using this library commonly transduce more than 1x10 cells 
As CRISPR continues to exhibit low noise and minimal off-target effects, an alternative strategy is to reduce the number of sgRNAs per gene for a primary screen. Less stringent cut-offs are used for hit selection, and additional sgRNAs are later used in a more specific secondary screen. This approach is demonstrated by Doench et al. (2016), who found that >92% of genes recovered using the standard protocol were also recovered using fewer sgRNAs per gene. They suggest that this strategy could be useful in studies where scale-up is prohibitively costly.