Integrating multiple stores (within a single Magento account) to a single NetSuite instance is packaged in the out of the box Magento 2 - NetSuite Integration App & comes at no additional subscription cost. You can connect an unlimited number of stores within a single Magento account.
Note: Integrating multiple Magento 2 accounts is different from integrating multiple Magento 2 stores within a single Magento account. For multiple Magento 2 accounts, see here.