have they said if it will be one game or trilogy

Nope they didn't say anything but in my opinion it will most probably have a sequel.Unless they decide to go the Dragon Age route and make each new game a self contained story with one protagonist which may or may not conclude itself with a DLC.

I remember reading something a while back about how a trilogy would work. It stated that in this first game we will only be exploring a portion of the Andromeda galaxy, most likely the Helios Star cluster, and as the series progresses it will open up more and more of Andromeda.

It's important to know that a star cluster can contain hundreds of thousands of stars. Our own Milky Way Galaxy has about 150 star clusters. Andromeda is believed to be bigger than the Milky Way and it may contain upwards of 500 star clusters so the possibilities are quite numerous and exciting. This is assuming that Mass Effect's Andromeda Galaxy is in some way related to the real Andromeda Galaxy haha.

I hope that unlike in the original Mass Effect Trilogy, if the latter games do return to places in the first game it will be much more rewarding and familiar.

On a side note, if you're interested in Science-y stuff like I am ShoddyCast did a great video talking about the ridiculousness of the science behind Mass Effect.

They haven't said anything one way or the other, but I doubt they will try to do a connected story across multiple games again. There are too many variables in game development to overcome and I think that is something BioWare themselves have made similar comments about.