I mean, I guess this is a valid complaint, but how many can you play at one time?

100 is too low

I don't want to play 100 at once :) How can you think that?
I want to install them.

Are you using Galaxy? Maybe I'm wrong but I think you can download multiple games that will install automatically with it.

But how to mark them? I can't use shift button or control or right click. So only way is install one by one :(

If the offline installers support command line options ("accept EULA", "close after install"), it would be possible with a batch file.

In Galaxy you have to select and install them separatly, but you don't have to wait for one to finish, the next one will be added to the download queue.

Hello, You're the first person I've seen request this. I doubt this is something the GOG Galaxy developers ever considered.

I think the best you can do is to first set the Owned Games category to List View. You then hover over a game, which displays the Down Arrow "Install" icon to the right of the game title. Click that for all games you're interested in. The first will download immediately and install when finished, while the others will get queued up.