There is. Välkommen is for greeting one person and välkomna is for greeting multiple people.
For something of neuter grammatical gender being welcome, e.g. "stödet är välkommet" ("the support is welcome").
Is this a Swedish custom to greet multiple people differently than greeting one person? I'm a native English speaker and in England we greet one person and multiple persons exactly the same.
Yes it is, at least when you speak directly in front of a group of people. When you are writing, you can use "välkommen" though you're directed to an audience made up of multiple (perhaps unknown) people.
How am I suppose to know how many people I'm addressing when all it says is welcome?
did you all know a secret- if you don't know the meaning of a word on duolingo, just hover and make your mouse stay there- you will see the answer of the question