It's weird that the authors decided to write '두개' (numeral and a counter) as a single word... While it is acceptable to write numerals and counters both with or without space in South Korea (in the North only spelling without a space is correct), it has been a standard for this whole course to write counters with a space in front of them
Honestly it doesn't matter anyway
I think this course has been space heavy, pronoun heavy and particle heavy because those patterns are easier to follow. Now I see in the final topics that they are making them less so so we can get used to how the language is actually used. However, there isn't really a means within the Duo format of truly conveying: comprehension of the particle endings, explaining complex case declensions to learners who have never even heard of the concept of cases, what it really means to be a topic marking language and what becomes irrelevant, and how natives actually pronounce Korean. Once I've gotten to the end of the Duo course I'm going to think about how to fill in the blanks.