Question
so, do I only deadhead the first flush of flowers and let the second go to seed so as to have more flowers next year?

Answer
Hi Heather,
Thanx for your question.  Yes, deadhead your lupines after they have begun to wither and a new flush of flowers will come.  After that flush, you can allow some of them to go to seed to ensure a crop for next year.  I take it you have annual lupines?  There are short-lived perennials too but most of these are cultivated.  I hope this helps.
Tom