Browsers impose a per-domain limit of 6-8 connections when using HTTP/1.1, depending on the browser implementation.
This allows at most 6-8 concurrent requests per domain.
With HTTP/2, browsers open only 1 connection per domain.
However, thanks to the multiplexing feature of the HTTP/2 protocol, the number of concurrent requests per domain is not limited to 6-8, but it is virtually unlimited.
It is virtually unlimited in the sense that browsers and servers may limit the number of concurrent requests via the HTTP/2 configuration parameter called
Typical limits are around 100 (Firefox’s default value for
network.http.spdy.default-concurrent – note the
spdy name here: it was the protocol ancestor of the HTTP/2 protocol) but could be larger (or, less commonly, smaller), depending on browser implementation and on the server you connect to.
Expect these limits to vary over the years with the evolution and the more widespread usage of HTTP/2 (in the same way it happened with HTTP/1.1: browsers started with 2 connections, and ended up to 6-8 after years of usage, experience and tuning).
I don’t think there is any difference between how a browser treats the number of connections and concurrent requests for normal browsing and for the usage of XHR, so the explanations above holds true for XHR as well.