Generally the only limitation for number of VPN tunnels is available memory, however on the Optimizations screen of the gateway/cluster object there is a soft limit of 1,000 concurrent IKE negotiations and 10,000 total tunnels that can be potentially increased. I've never needed to do so, and if you are bumping into either of these limits there is something seriously wrong with your VPN configuration, probably involving VPN Tunnel Sharing being set to "pair of hosts" and hundreds or even thousands of different hosts trying to communicate with each other through a VPN.
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