Advice on starting a second town

I've read the hints and tips section on the forums, but I'm curious to know what is the best number of settlers to use? 

Also, I have one settler so far, can I send that settler to start a new town, then send other settlers as they train?

IMHO, it's best to use as few settlers as possible.

When you click on settling button it opens window that tells how many you need to settle town

Clear enough? 

It's not possible to send settlers in that way, sorry

I've read the hints and tips section on the forums, but I'm curious to know what is the best number of settlers to use? 

Also, I have one settler so far, can I send that settler to start a new town, then send other settlers as they train?

Only 1 settler is required to start the second town and adding more makes no real difference.  The third town is another story.  The game could require 10 - 15 or more settlers, more or less depending on your kingdom size, so save up your settlers!  The settlers become more productive as they become classical settlers and as pilgrims, so you may need fewer.  Rather than sending more settlers, building houses to populate your new town is the best solution.

Thank you both so much. I already have 5 settlers, so it's good to know that I can keep 4 for the next towns. 

Although you only *need* one settler, the more settlers you have (and use) the sooner, and nearer you will be able to settle a town (e.g. if you have one settler, you might have to expand your kingdom by 50 cells, while if you have 10 settlers, you might only have to expand by 15 cells before you can settle a new town). I hope I've explained that well enough! However, as the others said, for your second town, you might as well wait until just 1 settler is all you need, as this will happen quite quickly. For later towns you're more likely to need/ want more, or more advanced, settlers.