The Rangecollection is the store for all Rows, Columns and cells and is indexed by the internal RangeID. The range ID is a long looking like this ...
((ulong)SheetID) + (((ulong)col) << 15) + (((ulong)row) << 29);
That not very user friendly, so I keep it internal.
The ExcelCell class is the internal representation for a cell and it could be public, but i did not want to have two interfaces for cells when I added the ExcelRange class so I set it internal as well as the Cell method of the ExcelWorksheet class.
To get the number of cells in the RangeCollection (I guess you mean the _cells variable), you would have to count the cells in the worksheet using a Linq query. Not very efficient, but if you have the need for it, it would be easy to add a CellsCount property
to the worksheet.