I clarified this with M&S last year when I built my latest bike. Any number of bikes with a individual value not exceeding £4k are covered no worries. The question I raised with them was around building a bike with a combined value of all the parts in excess of £4k but no single part more than £2.5k - this would NOT be covered as it would be termed a collection unless it was added as a specified item and additional premium paid.
Another thing to note is that the value of a bike (or sum of it's parts) is its RRP / replacement cost and NOT what you paid for it, i.e. if you have a £5k bike that you paid £4k for in a sale, this bike would NOT be covered unless you declared it as a specified item and paid an additional premium.
One more thing to note on the additional premium with M&S, you have to pay to insure the full value of the bike, not just a top up on the amount over £4k, so it does get quite expensive.