DMVPN - OSPF and multiple areas
OSPF should be split into multiple areas in order to make OSPF update messages smaller and to minimize the number of OSPF updates that may be sent over slower links. These factors have to do both with the scale of the network as well as the architecture and topology of the network.
One approach for DMVPN is to put one or more hubs you have into the backbone area, and then assign a different area to each spoke. This is useful when you have large networks with many prefixes behind each of the spokes. Another approach is to put the hub and spoke routers into area 0 and put the LANs that exist behind each spoke into non-backbone areas.
It is preferable to keep the whole DMVPN topology within a single OSPF area whenever possible since it eliminates the complexity that a multi-area OSPF topology introduces.
Take a look at OSPF design - when to create a new area for additional guidelines on creating multiple OSPF areas in various circumstances.