How many units of people should remain in the first settlement? Is it better to create more settlements very fast?
My personal strategy is quite specifically to remove the tools from the initial pioneer. Sell them to a nearby Indian village.
Then use this money in Europe to buy Trade Goods and repeat until you have enough money to satisfy getting of to a good start. As with towns I like to expand at the rate where I can leave at least one piece of artillery in every town from the get go.
That is I don't build a new town until I have artillery and at least one colonist in "surplus" in a nearby village. Secondly I usually don't try to build many more colonies than Indian villages close by as these are usually important resources in the early game.
Later on, this changes more to the number of towns I can reasonably defend and/or towns close enough that trade between them is reasonably quick (i.e. not traversing large strands of map).
It really depends on your circumstances, if you get more people than a master farmer or fisher can support it's my rule of thumb to expand, but if it's too dangerous to expand, or it would spread you too thin, then don't bother, play it safe. In early FreeCol games it's better to play cautious then to go all out, because early slip ups will cost you your game, I mean, unless you don't mind restarting of course!