It is easy to compute. You just need the ng of DNA input in the NGS experiment. For example, assume we input 250 ng of DNA (that is what is required in a standard WGS experiment*). Each cell contains 6 pg of DNA, hence, the number of cells is:
250 ng * (1000 pg / 1 ng) * (1 cell / 6 pg**) = 41,667.67 cells
*The amount of 6 pg per cell can be estimated from: human genome is around 6200 Mbp, the average nucleotide molecular weight is about 660 g/bp mol and Avogadro's number is 6.022 10^23. Hence:
6200 Mbp * (10^6 bp/ 1 Mbp) * (1 bp mol/ 6.022 * 10^23 bp) * (660 g/ 1 bp mol) * (10^12 pg/ 1 g)= 6.795 pg
Of course, as stated by ATpoint in the comment, the amount of input DNA highly depends on the NGS experiment.