Markers get bundles of 20 (occasionally slightly higher if the centre has say 22 students or there are 22 left at the centre they will make that one bundle). If your centre has 100 students then it is possible that 10 different markers will mark each essay e.g. 5 markers will mark the essay the first time and a different 5 get them the second time or one marker could mark a bundle from your centre for the first mark and get an different bundle from your centre for the second mark.
Anything marked online isn't marked in centres either I believe (I haven't marked that way but have spoken to people who have). From what I have been told they mark individual scripts and the next script that comes up is the one they mark which may or may not come from the centre of the one they have just finished.
Another comment about how many centres a marker marks - hard to judge as one marker might get a series of smaller centres of less than 10 (harder I know in English but still happens) while then marker next to them might only get sets of 20 - so obviously the first marker will appear to mark more centres but the total scripts that they mark will be similar.
Another factor is that markers also mark at different paces e.g. one marker might comfortably and accurately be able to mark 60 a night while the marker next to them might only do 50 but be equally accurate.