There are two important things about Terry Waltz that you need to know:
- she teaches Chinese to native speakers of English, so not everything she says about learning Chinese automatically applies to languages closer to English (or closer the native language of pupils or students), and
- she uses TPRS / Teaching Proficiency through Reading and Storytelling, which focuses much more heavily on comprehensible input than many other teaching methods.
I could not find a statement by Terry Waltz mentioning 50 exposures, but I found the following comment by her on a blog post (14.05.2010):
In classroom teaching using comprehensible input, we usually aim for 70-80 exposures in novel contexts during the first day of presentation, then regular "refreshers" when the item is presented in the course of input over future lessons. 10-20 isn't nearly enough, especially if the language is not highly similar to English. For a cognate, maybe. For anything else, not really.
So she admits that the linguistic distance between the native language and the target language impacts the number of required exposures, though it is not clear how much. The influence of the teaching method is not mentioned at all.
For a demonstration of teaching Chinese using TPRS, see the YouTube video TPRS DEMO: Day 1 Mandarin, Terry Waltz (18 minutes, 04.03.2015; notes that appear on the video explain what she is currently doing and why).
In an earlier response to a somewhat related question, I had found that 8 to 10 revisions were required. Fooziyeh Rasouli and Khadijeh Jafari wrote in their paper A Deeper Understanding of L2 Vocabulary Learning and Teaching: A Review Study (International Journal of Language and Linguistics, January 2016),
Webb (2007) explains that for each repetition of a word, at least one piece of word knowledge is acquired; therefore, a typical learner should meet a word about 8 to 10 times to obtain full word knowledge. What is worth mentioning here is the intervals between the repetitions. Nation (2001) refers to the conducted studies on memory and reports that "most forgetting takes place immediately after first encounter with new information. That is, the older the piece of knowledge, the more slowly it will be forgotten. This suggests that the first several encounters should be close together, with later encounters spaced farther apart" (p.24).
"8 to 10 times" is a number of exposures that, according to Terry Waltz, "isn't nearly enough", but I think the teaching method and, consequently, what exactly constitutes "exposure" make a crucial difference here.