I usually have less than 1 worker per city.... but... at least 2-3 more slaves for each city. I obtain slaves however I can, war, trades, extortion, etc.
We all know that slaves work slower than your own workers, but having more than enough of them will more than make up for it.
Ever since
ainwood posted up his
Civ Assist tool, I can't imagine playing civ without it. Everytime the AI civs have workers available for sale, I'm immediately aware of it and immediately trade to get them - hopefully, with the least financial hit. Although, the AI severely undervalue their workers, if I have the upper-hand, I would renegotiate the peace treaties, ROP treaties, territory/world maps, etc. But if necessary, I'd still be more than happy to pay the asking price in gold.
In my current game, I currently have 46 cities, but only 14 workers (that's all I've built since the start, none were lost or disbanded), and over 100 slaves.
Plus, with replaceable parts, the speed of slaves become a non-factor.