import arcpy

fc = r'C:pathtoyourfc'

startNumber = 51

with arcpy.da.UpdateCursor(fc, "myField") as cursor:
    for row in cursor:
            row[0] = startNumber
            startNumber = startNumber + 1
            cursor.updateRow(row)

import arcpy

fc = r'C:pathtoyourfc'

startNumber = 51
endNumber   = 75

with arcpy.da.UpdateCursor(fc, "myField") as cursor:
    for row in cursor:
        if startNumber <= endNumber:
            row[0] = startNumber
            startNumber = startNumber + 1
            cursor.updateRow(row)