I don't think there is a hard limit, however it can be too much to process all at once if you are using a microflow to import the data. I would suggest to process the data in batches.
When you imported the 1500000 rows of data, you should process and commit the objects in batches of maybe 5000 at a time, this will make it much easier for Mendix to process and commit the objects.
Hope this helps!
I agree with Matteo, There is no limit for uploading the data but if you try to upload bulk data in one go, you will end up with nothing upload with connection timed out. It is better to upload data in batches(you can split your excel file in 2 files and upload it one another) and also commit it in batch to maintain performance of your application