For the best results, I recommend using the green lentils as per the recipe. The lentils make up a big percentage of the burger and the recipe hasn't been tested with anything else. Possibly cooked beans would work ok but they will give the burgers a different texture and flavour.

Both after cooking.

Not a silly question at all! You need fresh beetroots for this. Canned won't work. They would be too soft and wet. As for how many you would need, it really depends how big they are. Sometimes they are teeny tiny and sometimes they are giant beasts so it's really hard to say. Usually here they come as a bunch so I just buy one of those and use what I need then do something else with the leftovers. I would guess it would probably be about 3 medium beets? If they are loose where you are and not bunches I'd buy 4 just to be safe. And you grate them and use it raw. That's what gives these burgers such great texture and stops them being really soggy like lots of vegan burgers tend to be.
If you end up with beets leftover because you buy too many, do yourself a favour and peel them, chop them into chunky pieces, toss in a tablespoon of olive oil, sprinkle with salt and pepper and roast them on about 400f for about an hour or until they are soft when you stick a fork in them. Honestly, they are the best thing ever! I hope that helps and you enjoy the burgers!

You're welcome! Usually my attempts at making make veggie burgers end in tears because I'm so frustrated that they won't stay together. Last night was tear free! :) Seriously genius on the butter as glue!

Oh that's awesome! You are right, the cashew butter acts like glue. I'm thrilled you enjoyed them. Thank you so much for letting me know!