As an English native (American), this looks and feels correct to me.
"You have until the 20th to complete your homework."
"You have until tomorrow to pay back what you owe."
"You have until morning to finish your paper."
"You have until the end of the month to catch up with your assignments."
There are several meanings for the word “sentence”. The online Merriam-Webster describes “sentence” as “a group of words that expresses a statement, question, command, or wish”, without restricting its grammatical form. But it also describes “sentence fragment” as “a group of words that is written out as a sentence but that lacks a subject or verb” (as the short version of the definition), thereby implying that a sentence proper has both a subject and a verb (predicate).
In a more grammatical approach, it is the presence of a predicate that makes a sentence. Many languages allow the omission of a subject, since the agent is expressed by the inflected form of the predicate. Even though this might be described so that the subject is present, but implicit, there are also many languages where sentences with no subject are normal, e.g. sentences corresponding to “It is raining” do not require a formal subject.
It is still a matter of definition what you call a sentence. And it is anyway a fact that people frequently use expressions that are not sentences in the grammatical meaning mentioned above (i.e. lack a predicate), yet serve the function of such a sentence. They are very common in short notices, news headings, etc. Usually the predicate can easily be inferred, as regards to the meaning, though we might have different ideas of the exact wording. But the point is that such wording does not matter.
“System shutdown in 60 seconds” can be read as “System shutdown will take place in 60 seconds” or in a few other ways, but the meaning is the same. Omission of the predicate probably makes communication more efficient here: the message is shorter and can thus be read and grasped faster.
However, such expressions are not accepted in normal flow of written nonfiction prose, or in “formal prose” as some people might say. The reason is that such forms of language have been developed to have a certain structure, including “complete sentences”, i.e. sentences in the grammatical sense. This makes them more readable, since this is the form we are used to expect. When you use sentence fragments, you break the normal flow. This may be an important technique in narrative text and even in nonfiction prose at times. But when used just because you don’t care to formulate your messages as normal prose, it makes the impression of… lack of care.
This is why Microsoft Office Word shows an error message in Spelling & Grammar checks, if you try to use such expressions. This can be changed by editing the settings. The way to do this depends on Word version, but the setting in Proofing options is “Fragments and Run-ons”. Even if this setting is checked (as it normally should), Word accepts fragments in many contexts. For example, if you write “Hello world” as a heading, or as paragraph of its own, it will pass. We can say that Word expects it to be a title or something like that and checks just the spelling, not the grammar. But if you end with a period, Word interprets that it is meant to be a sentence and says: “Fragment (consider revising)”.
Best Answer
You can't swap "51,000 people" (a plural) for "that" (a singular).
If you swapped it for a simpler pronoun instead, it stops working:
"This is what they looks [sic] like."
But it works because of the reasons in chasly's comment. You are referring to the entire crowd of 51,000 as a single entity. "This is what a crowd of 51,000 people looks like."
You could say "Here is what 51,000 people look like", but the emphasis is on what all the people look like individually, as opposed to what the crowd looks like.