Legal services by an individual advocate including senior advocate and firm of advocates to any business entity located in taxable territory attracts RCM.
However, an exemption is provided by law:
Services provided by individual advocate, senior advocate or firm of advocates to a business entity with an aggregate turnover of 20 lacs in proceeding financial year.
Business entity means any person carrying out business.
In your case RCM will be applicable if turnover of the charitable trust exceeds threshold of 20 lacs in PFY.