Should I use 2 relays, one for the hot wire and one for the neutral ? I'm talking about 16A.
Use 1 relay, more precisely a DPST (Double pole, single throw) type. You'll only need 1 signal to drive both, and be sure that both lines are switched off. Electrical installation regulations require double throw relays/switches for certain applications, e.g. in wet rooms like bathrooms.
Note that relays are more delicate devices than they appear to be. A 16A relay may not always be suitable for switching 16A. They have to be derated according to their type of load. And that is almost everything. Not only for capacitive or inductive loads you'll have to limit the current to a fraction of the rated 16A, but even for incandescent lighting (due to the high current peak when switching on). See the manufacturer's datasheet or application documents for the amount of derating.
As mentioned in the comments, if switching both, and the neutral relay fails open, unpredictable things may happen.
If you really want to switch both then use a two pole relay which switches both the live and the neutral together.
For instance, I use these relays:
20A, 250VAC contact rating, double pole.