I got an answer from IG, which doesn't really clarify the situation.
My question:
"The default limit for streaming API subscriptions is 40. The FAQ says that you are not allowed to have multiple concurrent connections. I'm wondering if I'm allowed to have multiple concurrent connections as long as my total number of subscriptions is less than 40, e.g. create 4 concurrent connections with 10 subscriptions each."
And the answer:
"The total amount of concurrent subscriptions per API key is 40. Therefore you would be able to concurrently subscribe to 40 different data streams.
The concurrent connections rule is so that you'd connect to the API using a single API key and application using a single connection rather than multiple applications using the same API key, meaning multiple connections.
Therefore it would not be recommended that you create 4 concurrent connections to have 10 subscriptions each."
