Assertion Even though net external force on a body is zero, momentum need not to conserved.
Reason The internal interaction between particles of a body cancels out momentum of each other.       [AIIMS 2019]

By Newton's second law of motion, net external force applied on a system is equal to rate of change of momentum.
i.e.  ${\mathbf{F}}_{\mathrm{ext}}\mathbf{ }= \frac{\mathit{d}\mathit{p}}{\mathit{d}\mathit{t}}\mathbf{ }$

If ${\mathbf{F}}_{\mathrm{ext}}$ = 0, then $\frac{d\mathit{p}}{dt}$ = 0 $\Rightarrow$ p = constant

i.e. When ${\mathbf{F}}_{\mathrm{ext}}$ = 0, then momentum of the system remains conserved.

The internal interaction between particles of a body do not contribute to change in total momentum of body.

Hence, both Assertion and Reason are false.