You cannot change the number of bureaucrats with this menu, the number is displayed for your information only. The percentage of workers devoted to government service is dependent on the tax rate you have specified. The higher the taxes, the more government workers required to collect and manage the money. A colony with no land tax will have 5 percent bureaucrats to control the economy and pass necessary laws. If you set a land tax of 20 percent, the size of the government will increase to about 20 percent of the population; the planetary economy will suffer accordingly.
As the landowner and titular head of government, you are entitled to a land-use tax, levied on each person in the economy on an annual basis. This tax is set as fraction of a man-year of labor. If you demand more than about 20 percent of the laborer's time, he will probably become angry, especially since many laborers at the low end of the scale only produce about 0.4 man-years of labor equivalent.
The tax is set aside as hard cash, and you can collect it with your ship at any time. Of course, if someone else captures the planet, he may collect your tax for you. Civil unrest may also diminish your tax receipts. Short-lived revolutionary governments almost always manage to spend or otherwise appropriate whatever possessions of the "bloodsucking fascist landowners" they can get their hands on. By the time the riot is over, no one remembers where the money went.