You can upload around 700,000 cells from one sheet at once.
Number of cells = (number of rows) x (number of columns).
The exact count also depends on the lengths of your strings and it’s impossible to tell what the exact uploadable number of cells will be in your specific scenario.
You should also keep in mind that we create a temporary sheet during the upload process. This operation has a temporary impact on the total count of cells in the spreadsheet. One spreadsheet can have max 5M cells at any given moment.
Example 1:
Your spreadsheet currently has 4.4M cells and you are trying to upload a sheet with 700,000 cells. The upload will fail because the spreadsheet would have to have 5.1M cells for a brief moment and Google will not allow that.
Example 2:
Your spreadsheet currently has 1.0M cells and you are trying to upload a sheet with 700,000 cells. The upload will likely go through because the max size of the spreadsheet during the upload will be 1.7M cells which is lower than 5M cells.
And that’s all!