Having some title and a holdership are two different things. I think going at this with an inflation of retconned elevations to holdership somewhen in the past isn't the right way. There is especially the difference between holdership and nobility to be considered: "Though all Holders are nobility, it is not true that all nobility are Holders. A holder's entire family, even distant cousins and aunts and uncles, are considered nobility, but only the title owner himself is considered a Holder." (I think it's a misconception that holders are called holders because they hold slaves, rather it's people who are in charge of a holding, that is who have land and the right to judge and make law in that domain. There were holders even before the Amarr started to practice slavery, after all.)
So, the number of holders matches the number of holdings, safe for destitute holders. As holdership gets usually passed on by hereditary means the chance to get hold over a destitue holdership is pretty slim, unless one gets adopted, first in line to inherit and the holder dies. All other freshly made holders should - imho - rather busy them selves to keep their holdings in order than playing capsuleer, as if one is raised to holdership, then this is because someone thinks you are able to look after that holding and expect you just to do that and prove yourself by doing so.
So, I think those Amarr, that don't have already the background as nobility fare better with the possibilities that CSCs offer. After all, if you have the trust of a holder he can charge you with buying/selling/educating/freeing his slaves in his name, which makes you fapp the one 'owning' the slave. (Just be careful not to sell his lordships favourite slave.
) If you're also a certified slaver, you can even buy and sell slaves in your own right (though prolly not free them, as I understood it).
I think it makes good sense that in the Empire legal ownership of slaves is quite restricted.
Also: A commoner can't be a holder, by definition. A commoner might be elevated to holder status - but than he ceases to be a commoner.