N.J. Stat. § 18A:16-1.1c
A vacant teaching position, or the position of a teacher who is absent for an extended time period, shall not be filled in any school year by one or more individuals employed as substitute teachers and holding a certificate of eligibility or a certificate of eligibility with advanced standing issued by the State Board of Examiners and working in an area not authorized by their credentials for a total amount of time exceeding 20 school days. The executive county superintendent of schools may grant an extension of up to an additional 20 days upon written application from the school district demonstrating the district's inability to hire an appropriately certified teacher for the vacant position within the original 20-day time limit. During a public health emergency declared by the Governor pursuant to the "Emergency Health Powers Act," P.L. 2005, c. 222(C.26:13-1 et seq.), a vacant or absent position may be filled by a holder of a certificate of eligibility or a certificate of eligibility with advanced standing and working in an area not authorized by their credentials for up to 40 school days and for an additional 20 school days upon written application to the Commissioner of Education or the commissioner's designee demonstrating the district's inability to hire an appropriately certified teacher within the 40-day time limit and that the individuals are periodically monitored and supported by the credential holder's supervisor.
N.J.S. § 18A:16-1.1c