How much do we need to pay for multi-piece submission under one company’s name? Will that be considered one entry? Or do we need to pay for each one?

Each design is considered a separate entry, unless they are part of a clearly linked series or campaign, such as a line of similarly designed kitchen appliances, or an ad campaign where the theme or look is the same, but the ad sizes are different. Each one needs to be paid separately, or by special arrangement with Spark.