  We study the number of limit cycles bifurcating from a piecewise quadratic
system. All the differential systems considered are piecewise in two zones
separated by a straight line. We prove the existence of 16 crossing limit
cycles in this class of systems. If we denote by $H_p(n)$ the extension of the
Hilbert number to degree $n$ piecewise polynomial differential systems, then
$H_p(2)\geq 16.$ As fas as we are concerned, this is the best lower bound for
the quadratic class. Moreover, all the limit cycles appear in one nest
bifurcating from the period annulus of some isochronous quadratic centers.
