About 3 or 4 out of 10 per batch are from non-arch backgrounds. For the most part, admissions make a point of assembling a class with a range of backgrounds. You'd have math graduates alongside English majors and former bankers, sculptors, pharmacists, journalists - the works.

They all showcased their unique backgrounds and expertise in their portfolios/applications. Everyone brings something fresh to the table, besides the requisite basic art and design skills - a good eye as evidenced in your portfolio, intellectual curiosity befitting Harvard's graduate schools, some semblance of design aptitude, the ability to put things together, an idea of communication skills (visual and written) ...

Worth noting that more folks from non-arch backgrounds do drop out, when they find out that design school is far from what they'd envisioned. For some with a strong quantitative or rigorous academic background, what passes off as theory and research in M.Arch seemed unworthy of their time and they quit. But most finished, and a couple even became the best designers of their cohort.

Thanks for the insight and advice :)

I'm a fossil but pretty sure the system hasn't changed much

3 background buckets

-architecture 

-fine arts 

-other

Portfolio is by far most important, even for those in the 'other' bucket.  They will make a decision on your port probably within 15-30 sec so need to be aggressive in design intent (it can look very raw, more important it looks visually compelling) and not worry about looking like a seasoned architect.