Это может помочь вам
import numpy as np
rname = ['A','B','C','D','E','F','G','H','I','J','K','L']
n_times = int(np.ceil(len(data) / len(rname)))
lst = np.repeat(rname,n_times)
n_elems = (len(df) - len(rname))
output = list(lst[:n_elems*n_times]) + list(rname[n_elems:])
print(output)
OUtput
['A', 'A', 'B', 'B', 'C', 'C', 'D', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L']