I've done many hours of research and while the top of the MAP score is 320, the individual subtest areas seem to ceiling around 260. There are still questions to answer after 260, but they are limited.
There are also three separate levels of MAP tests and a kid scoring at or near 240 on the 2-5th grade test, should bump up and take the 6+ test. In theory, the test continues but it's clear looking at the Descartes reports that the level of difficulty on the 6+ test is dramatically different than that on the 2-5th grade test. The continuum doesn't truly continue from test to test, though the RIT score is continuous. The continuous RIT score makes it easier for teachers and schools to compare the data.
When looking at percentile, I like to also look for what grade my child is performing at 95th percentile or above. NWEA uses 95th percentile+ for "gifted". If you have a situation where your child is 95th percentile for the next grade level up, in all subjects, it might be worth a conversation about a grade skip.