Hello,
There is no official data for the public in the datasheets regarding your question but I have pulled information from our internal test. RUT240 was able to handle 30 GRE tunnels at maximum. Note, that when approaching the client count limit, the router may start having performance issues and services might even crash. This is especially true for RUT240 because it does not have a lot of RAM, which is reflected by the low amount of clients.
Regards.