Hello Anthony,
be aware that in real world multiple options are possible.
First of all we should know if international offices connecting over VPN use the VPN connection to HQ for internet access (for more control) or they use the local handoff to access the internet.
Let's suppose that using the VPN tunnel to access the internet is desired.
The best idea to take advantage of OSPF hierarchy is to put the international remote sites in an OSPF stub area (under the hyphotesis above). Area 11
The 15 regional offices are probably using low end routers and they again can take advantage of OSPF stub area, in this case for maximum resource savings you could use a totally stub area.
To be noted every remote office will receive the routes of the other regional remote sites because they are in the same area even if it is totally stub. Area 22
An area 0.0.0.0 is always needed in multi area OSPF so it is reasonable to use 3 areas in total in this scenario.
to be noted if international sites use local links for internet access the area 11 should be a normal area or an NSSA area because it is not acceptable for them to receive a default route over the tunnel ( this is automatically generated on ABR nodes of a stub or totally stub area this is the key point).
On the other hand regional branch offices have no local internet option so they can benefit from a stub area in any case.
IF the MPLS service is a layer 3 VPN can be acceptable to put regional branch offices in area 0.0.0.0 because most of implementations using OSPF as PE -CE protocol use area 0.
So also 2 areas could be an acceptable solution.
Hope to help
Giuseppe
