In the decimal system we have
一 10^0, 十 10^1, 百 10^2, 千 10^3, 万 10^4
Remember, the power indicates the quantity of zero appears behind a single number.
We array these units when reading a big number, as 万千百十个 (where 个/个位 means single digit or the unit). For example, 96,128 is reading as 九万六千一百二十八.
A number which is bigger than (or equal to) 10^5, will be presented circularly by these units as 千百十万千百十个. For example, 75,896,128 is reading as 七千五百八十九万六千一百二十八.
THE KEY is, we use four digits as a loop for convenience, 千百十万,千百十个. We don't say 七千万五百万八十万九万, actually we omit the anterior units of 万 and hold only one to indicate.
万万 for 10^8 is not used today, commonly. We have 亿. And again, 346,275,896,128 is reading as 三千四百六十二亿七千五百八十九万六千一百二十八.
After that is 兆, the unit you asked. So it means 10^12. Thus, there are 12 zeros behind 一兆, as 1,000,000,000,000.
Here's a different way to show a number bigger than 万, and I think it may be the reason beset you and many people.
"十万为亿，十亿为兆，十兆为京." 亿=100,000, 兆=1,000,000, 京=10,000,000.
Then 75,896,128 is reading as 七京五兆八亿九万六千一百二十八.
The chaos comes, for we don't follow the original rule that each digit has its own unit. We use four digits as loop to reach 100,000,000,000 in the economic region on daily life. With the total amount of GDP in China rising over that much, 兆 will be used as a new current unit in the mainland. And the past problems of the units will still troubled people even then.