Having opened a TAC case (SR 687916345) the answer is: the ISR routers doesn't have a hardlimit:
Please note I went through our database, unfortunately The maximum number of areas is not fixed, it depends of the router capacity and the topology. The more areas you have on an ABR, the lower the performance of that router will be (especially if it has to handle big topology databases). The best way to design the OSPF network is to try to reduce the number of areas per ABR as much as possible
"Number of Areas per ABR
ABRs will keep a copy of the database for all areas they service. If a router is connected to five areas for example, it will have to keep a list of five different databases. The number of areas per ABR is a number that is dependent on many factors, including type of area (normal, stub, NSSA), ABR CPU power, number of routes per area, and number of external routes per area. For this reason, a specific number of areas per ABR cannot be recommended. Of course, it's better not to overload an ABR when you can always spread the areas over other routers. The following diagram shows the difference between one ABR holding five different databases (including area 0) and two ABRs holding three databases each. Again, these are just guidelines, the more areas you configure per ABR the lower performance you get. In some cases, the lower performance can be tolerated".
Therefore, it is recommended to have the least number of areas per ABR as possible. However, as there is not an easy rule to know which is the maximum (it depends on many factors: type of area (normal, stub, NSSA), ABR CPU power, number of routes per area, and number of external routes per area)
Here is a good links you can have a look
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html
http://www.ciscopress.com/articles/article.asp?p=1763921&seqNum=6
Factors Influencing OSPF Scalability
Scaling is determined by the utilization of three router resources: memory, CPU, and interface bandwidth. The workload that OSPF imposes on a router depends on these factors:
- Number of adjacent neighbors for any one router: OSPF floods all link-state changes to all routers in an area. Routers with many neighbors have the most work to do when link-state changes occur. In general, any one router should have no more than 60 neighbors.
- Number of adjacent routers in an area: OSPF uses a CPU-intensive algorithm. The number of calculations that must be performed given n link-state packets is proportional to n log n. As a result, the larger and more unstable the area, the greater the likelihood for performance problems associated with routing protocol recalculation. Generally, an area should have no more than 50 routers. Areas that suffer with unstable links should be smaller.
- Number of areas supported by any one router: A router must run the link-state algorithm for each link-state change that occurs for every area in which the router resides. Every ABR is in at least two areas (the backbone and one adjacent area). In general, to maximize stability, one router should not be in more than three areas.
- Designated router (DR) selection: In general, the DR and backup designated router (BDR) on a multiaccess link (for example, Ethernet) have the most OSPF work to do. It is a good idea to select routers that are not already heavily loaded with CPU-intensive activities to be the DR and BDR. In addition, it is generally not a good idea to select the same router to be the DR on many multiaccess links simultaneously.
