search and replace first amount of strings instances with one thing and a second amount of instances with another thing-

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i have a code in python to search and replace what i need though is to replace the first say 10 instances of the number 1 with 2 and the second 10 instances with the number 3. anybody knows how to do that?

fin = open(r'F:\1\xxx.txt')
fout = open(r'F:\1\xxx2.txt', "wt")
for line in fin:
    fout.write( line.replace('1', '2') )
fin.close()
fout.close()