Question is, how many should I use on a 4.2m span?

One either end and one in the middle?

According to BM Trada, that is acceptable, although it says this is a
minimum requirement and recommends you have more if possible.

Above 4.5m span, you should have 2 intermediate struts, so you are getting
into marginal territory for 1 intermediate with your 4.2m span.

http://www.luton.gov.uk/Media%20Libr...r%20floors.pdf

Christian.