Check in OAC3 which content repository is assigned to ARCHIVE document type, this should be the one that you see in SARA in case the CREP is empty in ARCH_USR
Number of objects is not necessarily number of records.
An object is a business case, let us keep the example of the sales document, so number or data objects would equal to the number of archived VBAK records, but there can be sales orders with just one item and sales order with several hundred items, not to mention that there are records for partners, records for long texts and many more.
It is tough to impossible to know how much byte one object will need.
if you would only go my number of objects then you could end with very small archives or very big archives. Especially with big objects you can get in trouble while storing because of disconnections. You can also get in trouble with available memory while creating indices.