From a physics standpoint, there is not much of an indication to whether there should be as many as 146 elements.
The Wikipedia article on Extended periodic table is a great read for this
Consider that Richard Feynman proposed that there couldn't be an element above 137. Since basically the electrons would have to move around the nucleus at or above the Speed of Light.
Now since then theories have been postulated that the actual upper limit should be around 173
146 does not have seem to have a real significance in any of these theories. Most notably element 146 would be in the same group as Plutonium is. In the 8th row.
In-Universe maybe they had a joint facility for creating heavier and heavier elements looking for the Island of Stability
When they reached element 146 they created some kind of Superplutonium that destroyed the facility immediately after being created.
Or: Element 146 is Naquadah and the pivotal element within the aforementioned Island of Stability and any Element above that has a Half-Life close enough to the Planck Time to be considered non-existent for all intents and purposes
Perhaps the choice of 146 had nothing to do with the actual number of elements the four Races had discovered.
It might have been chosen in regards to their actual inter species language that may have been comprised of 146 Symbols
The Ancients have been discovered to utilize the base 8 number system in the episode The Fifth Race
Well the Base 8 representation of 146 is 222. Kind of neat isn't it?
So maybe the creator(s) of the "language" were fans of Repdigits
Further noteworthy properties of 146 are:
... an octahedl number is a figurate number that represents the number of spheres in an octahedron formed from close-packed spheres.
An untouchable number is a positive integer that cannot be expressed as the sum of all the proper divisors of any positive integer (including the untouchable number itself).
So 146 could have been chosen for any combination of it's properties.
Note that, through the use of Wikipedia you will find interesting properties for almost any number! See the (humorous) Interesting Number Paradox for that. Or this Numberphile Video on the topic.