According to MSE licence "Microsoft Security Essentials can be used for small business up to 10 PC." But if my company has about 40 PC then what can i do? Can i have a option of purchasing MSE? Or is there any method to do it legally?
Unfortuanaly - not directly.
For business users Microsoft had other security solutions in the past (e.g. Forefront sales ended first december...).
Still you have multiple options:
With 40 PCs you can think about a centralized approach.