When you say you won’t be going anywhere else - do you mean that absolutely, i.e. for the duration of your holiday? If so, you just need a regular train ticket, not a Eurail pass.

For your Paris-Amsterdam journey, direct high-speed trains are run by a company called Thalys. You can buy directly from them at thalys.com, or from the French (SNCF) or Dutch (NS) national railway companies. These tickets will already include an obligatory seat reservation in the listed price.

Eurail passes are designed to cover rail touring itineraries lasting at least several days, and they are unneccessary and uneconomical for trips that can be achieved within a single day. When you use one to travel on a train requiring seat reservation (of which there are many in some countries), this has to be bought at extra cost as a separate item, because the pass already acts as your basic fare.

Hope this helps!