SQL Server Express limited by instance or by installation?

I've got one installation of SQL Server Enterprise and a couple of satellite installations of SQL Server Express. I can't justify additional installations of SQL Server itself for these satellites.

The documentation for SQL Server Express clearly states that it's limited to the lesser of one CPU or four cores.

What I can't seem to ascertain is whether this restriction for SQL Server Express really is per instance or per server. Some documentation uses the word "instance". Other documentation does not.

Instances don't seem to be much of an overhead (these days much of the SQL Server application is a shared installation, contrary to this older information). If this is an effective get-by to run several small-ish databases on the same server then that would be very convenient.

Can someone help clarify whether the limitation is per instance or per server, please?