I'm not at all sure quite what you're asking - how to plant a basket full stop? How many begonia specifically in the basket? I don't want to write an essay on basket planting if that's not what you want to know.

Hello Dalc :)
I just use multipurpose compost for my begonia baskets, and just pop in a couple of plants....they'll soon fill the basket, especially if you keep them watered and well fed (I use tomato food). Hope that helps....

The begonias will trail naturally so just plant the in the top of the basket.

Ah, there's been a bit added to the question - Beattie's right, just plant in the top, they trail over sufficiently to more or less cover the bottom of the basket.