This is explained on the same page. In the table it shows you that in the sandbox free tier, for each HTTP request to the Twitter API you can get a maximum of 100 Tweets back; in the premium paid tiers you can get a maximum of 500 Tweets per call. So for 10K Tweets you’d need at least 20 calls in the premium tier.
A period is a billing period, ie a month, so at the bottom tier you get 100 requests in a month.
If the Tweet you’re getting Retweets for is within the past 30 days you could also consider the premium 30-day search API instead.