Answer
$3$ neutrons
Work Step by Step
The given fission reaction is
$n^1_0+U^{235}_{92}\rightarrow Sn^{132}_{50}+Mo^{101}_{42}+n^1_0 b$
First we check the atomic number on both sides
$0+92=50+42+b(0)$
$92=92$
Now we check the atomic mass on both sides
$1+235=132+101+b(1)$
$\implies b=1+235-132-101=3$ neutrons
