Multi-part pastas and series are accepted, however they are only considered if you submit the entire, completed series at once.
The only exception to this rule is if it’s a series that sprang up organically from a stand-alone pasta; if you initially wrote your pasta intending it to be a one-shot and you were later inspired to continue the story, that’s fine. However, submitting “Blah Blah Part One” all on its own with the caveat “if this gets published, I’ll write more” will earn you an automatic rejection (we even have a category on Crappypasta dedicated to all the people who submit random, unfinished “part one” or “to be continued” pastas with no follow through). This is to prevent readers from having to deal with the annoyance of a bunch of unfinished series. Nothing sucks more as a reader than getting really into an interesting multi-part story, only to have the author abandon it mid-way!