Too many Partitions and Two "Data" Volumes

I open Disk Utility, that shows me 3 partitions:

They are not partitions. They are volumes which are sort of a virtual division of the APFS equivalent of a partition called a Container.

So, my question is, why exists two "Macintosh HD - Data" ?

You somehow reinstalled the OS without removing the old Data volume.

One of them is empty (disk1s1 - /Volumes/Macintosh HD - Data). Why? Can I delete it?

Yes, you can Remove the one that doesn't have the Home badge on it (second one in list). Select it and select the Remove Volume button.

Another doubt, the volumes disk1s6s1 (mounted at / ) and disk1s5 (mounted at /Systems/Volume/Data) seems the same directory. But I dont found any symlink appointed to other, except home folder (/home -> /System/Volumes/Data/home).

You can mount a volume anywhere any number of times--they are not symlinks. However, those two are not the same. disk1s6s1 is the same as Macintosh HD, but is the Sealed APFS snapshot of the system volume. You can read up on APFS snapshots, but they are just a virtually zero-space pointer to the data on disk. Not really a symlink.

/System/Volumes/Data is the mount point for Macintosh HD - Data (your data and third-party applications).

Finder merges the OS Volume and the Data Volume into one entity in the GUI, and you can find various locations that appear to be duplicates of other locations in the file system.