Lupin seeds are the size of a small pea, simply plant as many, plus a few spares, as you want to actually grow. Take a look at the website below for advice on sowing

https://www.westcoastseeds.com/how-to-grow-guides/grow-lupin/

I don't really understand what the problem is. One seed will produce one plant, so just sow as many seeds as you want - you don't have to use them all.

I can't help you with gardening in New Hampshire (I'm in the UK) but an internet search will tell you all you need to know.

The USDA zone for New Hampshire is much different from Miami's, which means it gets much colder where you are now - its worth knowing what USDA zone you're in so you can work out when to plant and what to grow - most plants mention the Zone/s they're suitable for.These Zones were recalibrated and updated in 1990 because of changing climate, so some zonal maps on the web aren't quite up to date - link below is to the updated version - you just need to key in your zip code

http://www.ahs.org/gardening-resources/gardening-maps/hardiness-map

You'll note that the link provided by Moon Grower above does give instructions for planting lupins which also contains the Zones they're suitable for, in this case, zones 1-9. I think your Zone is 3-4, but put your zip code in to find out for sure.