Contours: number of clusters for which weak-lensing mass estimates can be obtained by a Euclid-like survey. From outer to inner contours the lines represent isocontours of S/N > 5 weak-lensing detection of 1,10, and 100 clusters as a function of redshift and mass. Points: a Poisson realization of the above, with errors on mass and redshift (these move points outside the N = 1 contour).