Tax Offices increase the taxation on your planet through magical means.
The way tax offices (TOs) work is quite simple. Imagine having 200 buildings and 20 of them are TOs. Then the percentage of TOs is 10% of your total amount of buildings.
Every 1% of TOs gives you an income bonus of 2%. So with 10% of TOs you'll get an additional 20% income.
If your income was 100k its now 120k. If your income was 200k its now 240k. Etc.
Note: The more TOs you build, the more your maintenance goes up, the lower your income gets. So don't be suprised if you don't get the exact 20% bonus. (or whatever your bonus is).
Note: There is an optimum of TOs and LQs that depends on your strategy. 100% TOs won't work since there won't be any space for LQs left so there's no use for a bonus. And having 0% TOs doesn't work either when you aim for a TO income bonus. Experiment until you find a balance that suits your strategy - Common percentages for LQ:TO are: 70:30 or 60:40