Option order execution and retail priority

This question is for those who knows ins and outs of options orders execution mechanics.

There is something called "retail order priority" or similar. My understanding is that if a retail customer places a marketable order, market makers have some kind of obligation to take that order even if it is not profitable for them. Also, the size is limited and personally I could never get a fill for more than a singe lot.

I'd like to hear from someone who knows how this works from the market maker's perspective and whether or not a single lot is indeed the biggest order size that gets preferential treatment.

Thanks