Answer the question as truthfully as possible with the given context, and if unsure, say "I don't know"
Context: NAME_1 was born in Tokyo, Japan in 1950. He moved to London with his parents when he was 5 years old. NAME_1 started school there, and his father began working as a nurse at the hospital. His mother was a housewife, and he had four brothers.

Question: How many sons does NAME_1's mother have? Respond with more than 100 words.
user