The right answer is two generations.
5-Bromouracil has three tautomers that have different matching properties. The keto form is complementary to adenine, while one of the enol forms is complementary to guanine.
As a result, 5-bromouracil is likely to pair with both adenine and guanine depending on its tautomeric state at the time of replication, which introduces mutations as the replications. During the first replication the analogue will be incorporated in the DNA (in front of a T for example). At the second replication (second generation), the analogue will have a new base in front of it in function of its tautomic form (ie instead of having in front of a T it will have a C and a this moment the mutation becomes complete).
