@Daniel_Altiparmak: in addition to what @umair shared, I would like to mention you may want to use bench and its import-csv command to import large amounts of data from your legacy system (Data Import Tool is usually limited to handling no more then 5000 records per a csv file to process). Please review https://frappe.github.io/frappe/user/en/guides/data/import-large-csv-file for more details.
You may find it useful to review another forum thread (https://discuss.frappe.io/t/migration-from-v3-14/18497) where the similar case of migrating from a legacy system to ERP Next v.7 is discussed.