達する: reaching a summit and an expected membership threshold
Compare a climbing party reaching the summit in the evening with membership expected to reach one thousand, identifying place and quantity as targets.
The summit reached in the evening
登山隊は夕方に山頂へ達した。 reports the climbing party reaching the summit in the evening. 達する, read たっする, takes 山頂へ as the destination, with 達した reporting actual past arrival.
夕方に identifies the arrival time. The route and total climbing duration are unstated, so evening arrival cannot be converted into a departure time or elapsed duration.
An expected count of one thousand
会員数がついに千人に達する見込みだ。 says that membership is expected to finally reach one thousand. 千人に identifies the numerical threshold, and 見込みだ supplies the prospect.
Preserve the expected outcome rather than reporting an established current count of one thousand. 千人 is the expected membership threshold, not a stated enrollment capacity.
A destination and a numerical threshold
The first target is a summit, while the second is a membership quantity. Their shared 達する does not make a physical destination and a numerical target the same information.
The summit was actually reached, whereas the membership count is prospective. Read the past form and expectation together with the respective place and quantity.
What are the targets in 山頂へ達した and 会員数が千人に達する?
Show the answer and why
The first is the summit as a destination; the second is one thousand members as a quantity. Summit arrival is reported in the past, while the membership count is an expectation.