The likely response to this question, if asked, today would probably be - one; Frank Lampard OBE (1989-94), who amassed 106 caps for England.
However, a chance encounter outside his home led to Tim Lamb (OB 1959-69) getting in touch to ask the very same question, having discovered another!
Tim writes "The other day I was standing outside the front of my house when an old man walked down the lane. He stopped for a chat. It turned out that he had known the house as a boy, when a man called Ogilvie lived there. Ogilvie, he said, had played for England."
Searches of the internet and the school archives reveal that Robert Ogilvie (OB 1866-89) made one appearance as a defender for England in 1874, and was a member of the Clapham Rovers team that won the 1880 FA Cup Final. This match in 1874, as with Frank's in more recent times, was classed as a full international.
Furthermore, in an article about football by teacher Mr Rennie in the April 1954 Brentwoodian, another OB, R.S. Frowd Walker (1864-67), also represented England in the first match between England and Scotland on 19th September 1870 and then again on 28th February 1871, scoring the only goal in both games. These matches were not recognised as official international matches as only players [from both teams] resident in England were selected.
Of course, if anyone knows of any other OBs who have played football for England, we would be delighted hear from you and move the number up from two!