How many years does student finance fund you for?

It's length of degree + one year - number of years of prior study in HE. It's not a fixed amount like "maximum 4 years funding", for full time funding (for part-time funding it is, although it's 16 years total for PT funding as the limit).

Assuming you're applying to a course that is something along the lines of MSci Biomedical Science with Foundation Year (Placement) then you should be funded for the full length of the course if you have no prior study at uni before, except possibly during the placement year - not sure how those work. You will also have one extra year as you can see in the above formula - usually called the "gift year", which is there in case you need to retake a year, or change course after one year.

Note that if you are doing a standalone foundation course, then reapplying to a separate MSci course (and they have different UCAS course codes), then usually you will use the "gift year" for that foundation year. If it's all under the same course code then it should be funded fully as above though.

My apologies I really don't understand what you are saying. I am doing MSCI Biomedical Science in September with a placement and foundation year. What does that mean for me? As I cannot seem to find the answer sorry x

Then presumably the length of the degree as stated on the uni website and associated with the UCAS code for that course is 6 years, correct? In that case you will have a total of 7 years of funding available to you (the length of the course plus one year, since you presumably don't have any prior study at higher education/uni level).