You are limited to pre-booking one dinner and one brunch (if available) at Palo and Remy (if on the Dream/Fantasy). That's 2 dinners and 2 brunches total. Once onboard the ship, you can make additional reservations, if available.
As far as I know, it's been that way since 2008 (when we started cruising DCL).
It's been that way since 2000 when we started. It can be possible to book additional ones on board. On one or our cruise we used to go to the Palo bar (Magic) before dinner for a drink as you don't need reservations to go there. At least twice while we were at the bar we were asked by the bartender if we wanted to have dinner there since they had space. We didn't end up eating there because we'd already planned dinner with our tablemates but we could have so I know it's possible. It just depends on bookings.
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