Yes, It is applicable to your establishment as you said you have 20 regular staff, you should deduct ESI only from that 20 employees suppose if your company having temp staff i mean workers used for construction on site, you should not deduct ESI from thier wages but you need to have Workmen's Compensation Policy for those workers to secure thier life.
In case if you need further any clarification pls let me know.
Regards
Jatin.