Five thousand. Any higher number of thousands will repeat 'n' and/or 't' and any number higher than 999,999 will repeat 'l' (as in million, billion, trillion, ...).
The largest number spelled without any repeating letters is "one thousand." Each letter in this phrase is unique, and it represents a significant value without any duplicates in its spelling. Other large numbers like "eleven" or "hundred" contain repeating letters, making "one thousand" the largest valid option.
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Without repeating, 4. With repeating, 8.
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9875.
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