Maybe SM can answer this...
But we need to not read into 121 to much as who knows how the number system works.
Does it mean this is number 121 of the Xeno Organisms they have found? I doubt in context to Alien Franchise they have found at least another 120 different Black Goo related Organisms but the potential for how many life forms come from this Agent in the Urns is limitless.
It could mean simply they have detected 121 Alien life forms, on other worlds. Many most likely just basic life forms.
XX121 is just a number given to one of the Life Forms that have been encountered by Mankind so far.
At the time of Aliens the Xenomorph was the most complex life that Mankind had detected in the Galaxy (off course at the time they never covered the Engineers). But at the time of Aliens they never said they had never found life before. Just not complex life.
We have to remember the term Xenomorph simply just means a Alien Form or Alien Species and so a Life Form Alien to Earths.