You can ask Support.
Marketo uses a CDN and has migrated to more powerful processing. If you doubt you'd get 10k form submissions per second or per minute, I very much doubt you need to worry about load.
What you are more likely to experience is that 10,000 form fills creates a trigger backlog for a while if your campaign is attempting to process the responses.
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Just seconding Josh's answer: no, you will not have to worry about overloading the page.
In fact, pressing Submit has nothing to do with "the page" itself -- that's posting the form to the forms endpoint, and unless you're talking about the Thank You page, there's no additional LP load as a result of the post.
Technical note: even in the absence of triggers, on a technical level upserting data from 10,000 form posts truly simultaneously is impossible. This is true not just in Marketo, but in any distributed system. Data from all the posts might be accepted and queued for later insert roughly simultaneously (since that's an append-only step). But if data must be updated randomly throughout an existing database there must always be some degree of locking and serialization (db updates performed one after another, not at the the same time). Of course the total time taken could still be just a few seconds or minutes. But technically, very few things users perceive as "at the same time" are really happening without coordination.