When reservation is optional, a ticket does not guarantee a seat. A reservation, on the other hand, does guarantee a seat.
The Amsterdam - Berlin trains can be busy but it very much depends on the day and the time of day. So "in June” is much too vague. Busy times are Friday and Sunday afternoon/evening and around holidays, especially around long weekends such as the weekend of Ascension day (in May this year) or Whitsun weekend. The latter one is 4-6 June, so expect busy trains on Fri 3 June and Sun 6 June.
Booking a reservation is not expensive though. Just 3€ via ÖBB or 4€ (€5.30 in 1st class) via DB (choose “Seat only” for both). Eurail want 8€.
Note: On most days in June, the trains Berlin - Amsterdam don't seem to serve the main station (Hbf) but Gesundbrunnen instead. They also serve Berlin Spandau (as always). Depending on where you're staying, it may be better to get to one or the other (by S-Bahn or regional trains, also included in the pass). You can enter "via Stendal” to get these direct trains instead of connections with a change in Hannover. Or request a reservation from Berlin-Spandau and figure out where you're boarding the train later.