data=['1_aa',"2_bbb","1_xxx"]

prexs=["1_",'2_','3_']

data2=[[] for _ in range(len(prexs))]

for aaa in data:

    for index, prex in enumerate(prexs):
        if aaa.startswith(prex):
            data2[index].append(aaa)

for data_item in data2:
    print(len(data_item))