You start off by saying this:
I have 4.361 units in a xml file, I may see all of them with NotePad, but when I open the xml file (created with Trados Studio) I may see only 50.
This suggests you are opening an xml file in Studio for translation, expecting to see 4.361 units parsed into the editor but only see 50. This being the case you probably need to create an xml filetype for this xml file and make sure that the parser rules are relevant to the specific tagging of the file. You do this before opening the file or adding it to a Project and would use:
Tools - Options - File Types - New - XML
Then follow the wizard to create your new filetype.
However, you then say:
I can´t find how to change the configuration to be able to see all of the units in this memor.
This suggests what you really mean is that you have a memory you have upgraded, maybe from TMX, and when you view it in the Translation Memory Maintenance window you only see 50 units. So, if this is the case you can increase this to 200 units at a time by going to:
Tools - Options - Translation Memories View
Then increase the number of Translation Units per page from 50 (the default) to 200.
You can also use the navigation buttons on the Translation Memories Maintenance view to navigate through the pages, 50 (or whatever you set in the previous step) at a time.
Hopefully one of these ideas helps?