What is the greatest number that will round to 900?

Assuming you are interested only in whole numbers, then it depends upon the nearest to which the number is being rounded:

To the nearest 3 or 4 it is 901

To the nearest 5 or 6 it is 902

To the nearest 9 or 10 it is 904

To the nearest 12 it is 905

To the nearest 15 it is 907

To the nearest 18 it is 908

To the nearest 20 it is 909

To the nearest 25 it is 912

To the nearest 30 it is 914

To the nearest 36 it is 917

To the nearest 45 it is 922

To the nearest 50 it is 924

To the nearest 60 it is 929

To the nearest 75 it is 937

To the nearest 90 it is 944

To the nearest 100 it is 949

To the nearest 150 it is 974

To the nearest 180 it is 989

To the nearest 225 it is 1012

To the nearest 300 it is 1049

To the nearest 450 it is 1124

To the nearest 900 it is 1349