As per the docs:

The http2.getDefaultSettings(), [...] APIs either return or receive as input an object that defines configuration settings for an Http2Session object. These objects are ordinary JavaScript objects containing the following properties.
[...]
maxConcurrentStreams <number> Specifies the maximum number of concurrent streams permitted on an Http2Session. There is no default value which implies, at least theoretically, 2³¹-1. streams may be open concurrently at any given time in an Http2Session. The minimum value is 0. The maximum allowed value is 2³¹-1.

$ node
Welcome to Node.js v12.10.0.
Type ".help" for more information.
> http2.getDefaultSettings().maxConcurrentStreams
undefined