If set B = 1,2 write down the number of subsets in set B.

The given set is $B = \{1, 2\}$.
The number of elements in set $B$, denoted as $n(B)$, is 2.

The number of subsets of a set with $n$ elements is given by the formula $2^n$.
In this case, $n = 2$.

The set $B$ has 4 subsets.