Can you hide your number of cards in Uno?
The official rules of most games tell you what is allowed, and usually only tell you what isn't allowed when it seems that it would be possible otherwise.

For instance, there is no rule that says I can't add four cards to your hand whenever I feel like it. (feel free to add your own ridiculous rule here)

Should this be allowed because the rules don't specifically prohibit it? No! Of course not! The rules can't possibly list everything that's not allowed.

It's an important part of Uno that you can easily tell how many cards your opponents have so that they can be penalised for not calling "Uno" on their last card. The only way they can get out of this penalty is if the other players don't say anything about this before they can play their last card.

So, no, while it isn't specifically prohibited, it's not legal to hide your cards to gain an unfair advantage.

Of course, if you have a house rule that says you can do this (and all the players have agreed before the game began), it is allowed. However, you're not truly playing Uno anymore, just a game that's very similar to Uno.