As far as I know, Esperanto is the only language without irregularities in its numbering system.
English is not too bad, except for:
Never sure how to pronounce 0 (nought, zero, nil, nothing, ow...).
Irregular numbers from eleven to nineteen.
Specific names for twenty/thirty/.../ninety.
Hyphen between the tens & units.
Adding "and" after the hundreds (six thousand, six hundred and sixty-six).
Ambiguity about what "billion" means. (How can anybody tolerate ambiguous numbers?)
French is worse, including:
Seventies are expressed as "sixty-ten", "sixty-eleven" etc.
Eighties as "four-twenties", "four-twenty-one" etc.
Nineties as "four-twenty-ten" etc.
German includes major glitches like:
Putting the units before the tens, "Six hundred six & sixty" etc.
Obviously, a new & easy-to learn language has to have absolute regularity & simplicity for all its numbers.
The base numbers should be monosyllabic & very distinct, for instance:
nil - won - tuw - tri - for - fiv - six - set - okt - nin - ten
Next groups up could be:
hunda - mila - miliona - ?
All numbers should be pronounced from left to right, with no joining up, no hyphens (that's questionable), no plurals & no "and".
Going against my mantra of simplicity above all, I suggest breaking up big numbers with a comma after thousands, millions etc.
I would use the same comma in the alphabetical & numerical representations.
It would introduce a clarifying pause in the spoken versions, as well as the obviously-necessary break in the written numbers.
11: ten won
12: ten tuw
20: tuw ten
21: tuw ten won
36: tri ten six
99: nin ten nin
101: hunda won
160: hunda six ten
199: hunda nin ten nin
200: tuw hunda
206: tuw hunda six
266: tuw hunda six ten six
999: nin hunda nin ten nin
1,001: mila, won
1,234: mila, tuw hunda tri ten for
2,000: tuw mila
20,000: tuw ten mila
20,001: tuw ten mila, won
23,456: tuw ten tri mila, for hunda fiv ten six
234,567: tuw hunda tri ten for mila, fiv hunda six ten set
1,000,001: miliona, won
2,345,678: tuw miliona, tri hunda for ten fiv mila, six hunda set ten okt
I will cover very big & very small numbers, & ordinals, separately.