Adding new players should be a group decision
When you form a gaming group and begin playing a game, the DM often gets some sort of authority given to them by the game. However, the DM does not (and should not) run the gaming group — the group does. They have just as much (but no more) say in group-affecting decisions as every other member of the group.
Just because someone is the DM doesn't automatically give them the right to add players without consulting the group just as it doesn't give them the right to arbitrarily change the location and time of game night, the type and presence of food at the table, or any other out of the game details.
Now you could give the DM this authority, but that would have to be something the group cedes to the DM, not something automatically given to them without discussion. If you haven't had that discussion explicitly, then they can't and shouldn't be making this (or any) decision for the group.
You should talk to the DM about this as soon as you can, not just for the sake of your current group size, but because a DM that thinks they can add players at will without asking may have a completely different view of their role at the table than you and the rest of the group.
I've been in the position of DM and player both as a person suggesting someone new to the table and as someone who was concerned about another proposal to add a player. So, I speak with experience when I say that discussing this as a group is the way to go (and that not talking with the group is asking for trouble).
We've had some times where discussion with the group brought up and nipped some serious issues in the bud. For example, one time the DM wanted to invite a player that not only was (unbeknownst to them) extremely insulting to several other members of the table, but would also have introduced logistical issues that would have negatively affected everyone. In the end, we talked it out as a group and decided not to add the person. I firmly believe that had the DM added this person without consulting us not only would it have been a huge breach of our trust, but it would have killed our group.
We've also had much more positive experiences where everyone talked over and decided to let another person in.
Large groups are not easy
Completely aside from the issue of who gets to decide who plays at the table, the number of players can drastically affect how the game goes. Larger groups amplify many of the problems that small groups already experience and can bring some completely new ones to the table as well. If the players and DM aren't experienced this can (but doesn't necessarily have to) be a recipe for unfun to happen.
I've played with more and I've played with less. There's no perfect number for every table, but you have a right and foundation on which you can base your concerns about adding people and you should definitely bring them up to your DM.