As per my knowledge, any non-seasonal factories using power and employing ten (10) or more persons, or non-seasonal and non-power using factories and establishments employing twenty (20) or more persons, are eligible to register for ESIC. Additionally, every establishment which is a factory engaged in any industry specified in Schedule I and in which twenty or more persons are employed, and any other establishment employing twenty or more persons or class of such establishments which the Central Government may, by notification in the Official Gazette, specify, in this behalf are eligible for EPF registration. If you go through the document file attached here by me, it is clearly mentioned there (Page 7, 11).
Regards