For example, we have name "Michael Daemon" - with 1 space. How we can mention this name via "@"?
If you start typing @Mich the auto-complete suggestion will appear*, and it will be @MichaelDaemon, without a space. It is a standard practice to accept the autocomplete suggestion.
The complete rules of matching names to users via @-mentions are described in How do comment @replies work?
(*) Sometimes it does not, and this is usually because you can't reach that user (they have not participated in the comments), or because mentioning them is redundant (they are the author of the post being commented on). See the reference above for details.