data = loadtxt("1.txt", float)

>>> '{0}.txt'.format(1)
'1.txt'

for i in range(50):
    data = loadtxt('{0}.txt'.format(i + 1), float)

for i in range(1,51):
   data = loadtxt('{0}.txt'.format(i),float)
   #process data here.

datafile = '%d.txt' % (i)