If you take climate change stuff seriously, then the answers are these:

>How many?

As many as you can build. You run out of engineers and carpenters before you make too many reactors in time.

>What timeframe?

Now.

>Costs?

Doesn't matter that much. Easily cheaper than climate change.

>Who builds them?

Whoever can.

>Which models?

Hardly matters, any model is safer than burning coal.

>Where do we build them?

Doesn't matter. Any location is safer than burning coal.

>What effects does it have when?

Hope that we might have climate change to stop below 3 degrees.