Japanese groups numbers into groupings of 4 instead of 3 like in English.
In English we have ones, tens, hundreds, and then they are grouped together and those are repeated for each section, each grouping of 3 gets it’s own name, thousand, million, billion, trillion, etc.
Japanese has 一、十、百、千 as the base numbers that get repeated, and then they are grouped by section, the first section is 万 the next one is 億 (hundred million). Think of it like putting the commas in different places in the numbers
If you speak English or any other language that groups numbers in threes, it would be natural to assume that 十千 is a valid number, but it’s not how the counting system in Japanese works.
English:
1 - one
10 - ten
100 - one hundred
1,000 - one thousand
10,000 - ten thousand
100,000 - one hundred thousand
1,000,000 - one million
10,000,000 - ten million
100,000,000 - one hundred million
1,000,000,000 - one billion
10,000,000,000 - ten billion
100,000,000,000 - one hundred billion
1,000,000,000,000 - one trillion
Japanese:
1 一
10 十
100 百
1000 千
1,0000 一万
10,0000 十万
100,0000 百万
1000,0000 千万
1,0000,0000 一億
10,0000,0000 十億
100,0000,0000 百億
1000,0000,0000 千億
1,0000,0000,0000 一兆 (pronounced いっちょう)
NOTE: this is not how comas are actually used when writing numbers using Arabic numerals in Japan, it’s just a way of thinking about it.