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The reason that there are different views, as possible answers for both your questions, because there are many variables involved. Some of the important variables include: how "powerful" a control plane processor the OSPF device has, the number of links within the area, the number of routes, topology stability, how OSPF is configured, and the actual OSPF implementation. Additionally, for Cisco OSPF, there are some tuning options that impact OSPF operation.
Some old rules of thumb use to be no more than 3 areas on an ABR and no more than fifty "routers" per area, but on modern equipment, these can often be safety exceeded, even by quite a bit.
