Assistants: two or more people who are helping
His assistants were busy preparing the report.

Assistance: the help being provided
I really appreciated your assistance yesterday.

The use of -ants and -ance is often confused because they sound exactly the same despite the very different spelling. The easiest way to remember which is which is to think of ants (yes, the insects!) as doers and use that ending when you are writing about people doing something.