If you want to use fresh spinach, go for about 100g and remove any larger stems.
Using the heat from the soup to thaw the spinach has two purposes- it brings the soup down to an edible temperature, and it keeps the nutrients of the spinach largely intact. And the color stays nice and green.
Sometimes I don’t add the lemon-if the brand of tomatoes you’re using is very acidic, you might want to leave the lemon out.