How would you record a drift of many plants e.g. bluebells which you couldn't possibly count?
The Abundance field for records is where you can tell us how many of each species you saw (in numbers or words). The default setting is 1 so if you don’t change it we will assume you have seen just one specimen. It's an optional field but abundance is very useful to know. If there are too many of one species to count, you can just write "Many" or "Very abundant" or whatever to give an idea of the frequency.
Hope that helps.