  Nonlinear Compton scattering (NCS) and nonlinear Breit-Wheeler (NBW) process
are strongly multi-photon and highly nonlinear processes. In ultra intense
lasers (normalized field amplitude $a_0 \gg 1$), radiation formation length is
much shorter than a period and single NCS/NBW cannot be described as
scatterings of electrons dressing plane waves with $\gamma$ photons for what
they feel is a local constant crossed field. However, present theories in
constant crossed fields are hard to give some important quantum features due to
divergence problems, such as number of laser photons involved, instantaneous
angular distribution and detailed spectrum. As an alternative, present
understanding of single NCS/NBW in ultra intense lasers includes several
classical and semi-quantum ideas such as forward emission, recoil reaction and
spectrum cutoff. We investigated multi-photon effects on NCS/NBW in ultra
intense lasers by extracting the number of laser photons involved in a single
process in ultra intense lasers from formulae of existing theories. New
features of single NCS in ultra intense lasers including fixed emission angle
to instantaneous electron momentum, instantaneous deflection of electron, and
disappearance of spectrum cutoff are deduced. Similar features of single NBW in
ultra intense lasers including non-vanishing emission angles to instantaneous
$\gamma$ photon momentum, disappearance of spectrum cutoff and appearance of
spectrum lower limit are also obtained. Simulations show that corresponding
signals of multi-photon effects are significant on $10$PW scale and stronger
lasers.
