In OpenGL Reference Manual it is said that in different implementations there may be more than one error flag. I’m confused with the word “more”. Is there only one error flag for each context? or Can there be implementations allowing more than one error flags for a context? for Microsoft’s Opengl 1.1 driver I could not get more than 1 error at a time (in a loop).

I think they mean that errors can come from other systems that OpenGL use. For example, on windows wglGetLastError would return errors from the window sub-system. glX uses some other error reporting. Look at the specs.