At a particular time? Or do you want the largest figure at any time?

It should be said that there are methodological problems in defining Christian. Does a European who says "I believe in God, probably, but not really in the stuff in the Bible, so I'll put Christian on my census because I know I'm not a Muslim or Jew", count as a Christian? On the other hand, a number of South American states tend to say "basically everyone is a Christian here", but in fact, most of those people may not be very religious. So, what defines a Christian exactly? Also, which denominations count? Are Mormons Christians, for example? They consider themselves to be, but most of the larger Churches do not consider them to be Christian. Then there's the fact that some states simply do not measure religion. And of course, there are "hidden" Christians. In China, a large number of Christians (perhaps the majority) are unregistered. In some countries, it's completely illegal. And that's just in the modern day, which has better statistics than the past.

Now, saying that, you could probaby get a very vague figure for some time periods, if you're willing to accept that. Currently it's estimated to be at about a third.

According to the annual studies by The International Bulletin of Missionary Research, the number has stayed at about roughly one third since 1900 (and other sources tend to agree). Before 1900, there was likely a much lower percentage of Christians in the world. The 19th century saw a huge effort by western missionaries to spread Christianity around the world, which had a slow but extremely effective influence on Africa (much less so elsewhere, but it did have some effects, for example in Korea, Vietnam, the Burmese hills, etc). Africa went from being, in 1800, a continent with a fairly low proportion of Christians (mainly in Egypt and Ethiopia), to being a 40% Christian continent today. (Though it should be said that many African religious leaders claim that it's not Christian enough, as many Africans follow traditional beliefs with Christian syncretism tacked on, or just go through the motions. I don't know how true that is, but it shows you the methodological problems of measuring religion again.)

One final note: in 1900, Europeans were (according to IBoMR again) 66% of Christians (Americans, Canadians, etc, being another large chunk). Today Europeans are 23% of Christians. Broadly speaking, Christianity has fallen sharply in Europe and the former Soviet Union (though less so in the former USSR than western Europe), risen slowly in Latin America (which was mostly Christian in 1900 anyway), declined slightly in North America, risen quickly in small and specific parts of Asia (for example, South Korea) with a smaller increase in other parts (like China), and exploded in Africa (both through conversion and population growth).