You are limited to 20 records because this is what is returned by your data source by default, so the formula has access to only these results. We haven’t implement yet a way to configure the page size returned by a data source.
However, I think you need the Record selector element instead. Don’t search for it yet, this element will be part of the next release (~ 1 month) and allows you to select a data source as source for your options. It loads the options as you scroll. Later you’ll even be able to search through the available options.
A sneak preview of the element:
You are too quick for us, but it’s coming 