Tournament-legal to use a d20 to represent amount of similar tokens on the battlefield?
You would have to make it clear which of the tokens are tapped and which are not. It is both players responsibility to keep a clear board state. It also has to be apparent what the size of the tokens are (1/1), the color (Colourless in this case) and any abilities that the tokens may have.

When you say 'tournament-legal' I assume you mean competitive REL. At this level of play, BOTH players are required to keep fair and honest board states. 

As a general rule of thumb, MTG rules want the best player to win the game, not those who by some happenstance made the better sense of a confused board state.

To come back to the question at hand, get yourself maybe only two servo tokens. One you turn sideways to CLEARLY indicate that these are the tapped tokens and have some that are upright to indicate that those are untapped.

You can use dice to keep track of the numbers or some sort of glass pebble also works well. Make sure to keep track of the numbers and if your opponent asks you a question about the tokens give him/her the correct answer.

If you willingly mislead a player to gain some sort of advantage, not only may there be some sort of reprimand, you will also very much lose the respect of all of your fellow players.