Which LN volumes are covered in season 1&2 of "Konosuba" anime?

According to Japanese Wikipedia:

• Volume 1 covers Season 1, Episode 1 (S1E1) - S1E6
• Volume 2 covers S1E7 - S1E10 and S2E3
• Volume 3 covers S2E1 - S2E5 except S2E3
• Volume 4 covers S2E6 - S2E10

So, you might want to start from Volume 5.

However, take a moment to read Is the Konosuba anime a close adaptation of the light novel? to decide whether you want to read from the beginning instead.