Hey!
I'm also having a bit of trouble putting together a good portfolio for De Montfort, but I do have a bit of experience coming off an Art foundation and a bunch of portfolio advice sessions and emails to tutors. From what I gather 20 pieces is the ideal amount, though you can have multiple pieces on each page so don't worry if you think you can't show enough in one piece or have a set of work relating to each other. So your gonna want life drawing I would aim for 5 pieces of life drawing, with each showing different times/levels of detail (so you could have one thats your 30 second studies and another that a 5 minute sketch for example). Your then gonna want some work from school/uni/college to show what you do academical and how you think in that environment; ideally you'll want to show a project that relates to Games Art, but at the same time anything relating to design/art should be fine as long as it shows good development and skill. And then you'll want 5 pieces of environment/prop/vehicle designs and sketches; this is fairly open so you can have models, sketches, paintings anything of these things; University's know that this is the section of work that people need work on usually (as most students have skill in character drawing rather than environmental) so just try and put it your best work. The last 5 can be digital work from models to digital paintings to just sketches, mainly your personal work; what you like to draw and how you do it in your own time.
So to summarize: 5 life drawings, 5 school/uni/college projects, 5 environments/props and 5 personal works/digital. Of course this is just a rough outline and should not be taken too literally, for example if you have lots of good life drawing but not as many sketches of buildings or whatever, then feel free to go over 5. This is just a rough outline that I'm working with that has helped me before when looking at courses.
Some general tips: A1 drawings is recommended by a lot of A-level teachers and is fine for getting into foundation or Fine Art, but you don't have to worry about it too much for Games art, so when putting your portfolio together don't worry about have some pieces that are smaller, as its about the content. For digital work you will be sending it to the school in a PDF or showing it to them in an interview on screen; so keep that In mind when deciding on how big your digital stuff should be- If your drawing digitally I would aim for a canvas of 2000 by 2000 pixels with 200/300 dpi, this is a large canvas that can be scaled down if needed, its always safer to go big then scale down rather than just working small. And final tip is: Sketchbooks are also important, when going for interview and even when sending work online to them, they will want to see your sketchbook to see how you study, research and think- its important; so if you think your sketchbook(s) is lacking then get to working on them.
I'm gonna attend a portfolio advice session for De Montfort next week, so if I get any information that seems useful I'll post it here. Good luck guys!