When you upload contacts from a CSV file via Contacts > Options >Import Contacts. The limit the system sets for an individual import is 60K contacts. But for ease of processing, we recommend you stick to 50K and under. So for example, If you have an exceptionally big file to import i.e- 90K contacts to import. You will need to split it ideally into 2 files of 50K and 40K and upload them from there.