why did the british needed a large army ?

Dear Student,

The answer to your question is as follows.

The British needed a large army to secure their interests and fulfill their objective of achieving territorial control over areas where they conducted their trade. They needed to defend their factories and trading posts and expand their trade. Achieving political control was necessary to fulfill this aim. The Company proclaimed itself as paramount during the time of Lord Hastings and strived to maintain this position in the face of challenges by local and regional rulers. External threat from Russia also required the maintenance of a large army to control the north-west.

Hope this provides a solution to your question.

Best Wishes