Do you mean, in the interest of precision, "What is the maximum number of lines in a file imported via the Marketo UI?"
If not, pls clarify your question.
In theory there is no limit.
Practically, over 1Millions leads, you need to start paying attention to performance optimization (and even above 700k, from my experience, if you are running a lot of smart campaigns).
Adobe is pushing Adobe Campaigns rather than Marketo for customers who need to have 10 millions + leads in their database.
Keep in mind that the licence cost is directly linked to the number of leads in the database, though.