Multi-instances cluster and ASM max lun numbers

I have an interesting situation: a three nodes cluster (using oracle grid infrastructure) hosting about 20 instances. The cluster is acting as a normal failover cluster, each instance has 3 diskgroups (DATA, FRA, REDO) plus a lun for binary.

This configuration is not usually described afaik even if it is working. An interesting thing is that ASM has a limit about max lun mounted: 64.
With this configuration there is a huge limit, in case of a node failure both nodes might not be able to support everything. Also we have seen that when ASM have to mount 10 luns in the same time it is very slow.

So, I am curious to understand how you would tackle with this problem and if you might use a different approach. Contraints:
- RAC is not available due to money limits and too many applications
not certified
- Instances cannot be aggregated

So any feedback is appreciated.

Thanks
Stefano

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