You can get item counts of a SharePoint list by SPList.ItemCount or SPList.Items.Count property. But these two numbers can be different. The inconsistency is due to the SharePoint content security. SPList.ItemCount shows the exact item number, but SPList.Items only returns the list items that current user has access to them.
For example, if a list has 10 items, and only 5 of 10 are accessible to you, then SPList.ItemCount = 10 and SPList.Items.Count = 5 in your security context.
In addition, SPList.ItemCount is more efficient than SPList.Items.Count, because SPList.ItemCount is just a property of the list, but SPList.Items.Count will result in loading all list items (with permissions) into memory.
