How to numerate formulas in Lyx? Is there exist LaTeX command like "\tag "? I haven't found corresponding info before.
When you say formulas, I assume you mean formulas in a display:
Insert > Math > Display Formula (there is a keyboard shortcut, but I don't remember what the default is). These are LaTeX
\[ \] environments, and become
\equation environments upon labeling or numbering.
Once you have inserted your display, you right-click the display and may from there choose to number it or label it for cross-reference. Again, keyboard shortcuts exist.
PS. If you want an
Shift + Enter with the cursor in the display, then number lines (e.g. by right-clicking).