Remember which spelling by using ‘dependent’ in much the same way you use ‘independent’ (as an adjective). I am dependent/independent; she is a dependent adult / an independent adult. In these examples, dependent – just like independent – is spelled with an ‘e’.

Then contrast these uses:

Our family includes five dependants: one teenager and four dependent adults.

(The good news is they’re all getting highly trained, so they can keep us in the manner to which we’ve become accustomed, as they say.)