More Numbers with Ascending Digits

Miri writes down all the numbers whose decimal representation contains only the digits `1, 2, 3, 4, 5, 6`.
(Not all digits must appear) and all the digits that appear are written in ascending order (e.g., 1356 or 124 or 5 but not 162 and not 1223).
How many numbers will Miri write down?

Difficulty level (1 very easy - 10 very hard): 1

Topics:
Combinatorics -> Product Rule / Rule of Product Set Theory
Sources:
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