Yes, it is applicable to your establishment as you said you have 20 regular staff. You should deduct ESI only from those 20 employees. Suppose your company has temporary staff, meaning workers used for construction on-site, you should not deduct ESI from their wages, but you need to have Workmen's Compensation Policy for those workers to secure their lives.
In case you need further clarification, please let me know.
Regards,
Jatin