Use & (ampersand) in single line bash loop

Drop the ; after &. This is a syntactic requirement

for((i=114;i<=255;i+=1)); do echo $i > numbers.txt;python DoMyScript.py & done

Given Stephane's comment on 1_CR's answer, you probably want:

for i in {114..255}; do { echo $i > numbers.txt && python DoMyScript.py; } & done

Lose the ;:

for((i=114;i<=255;i+=1)); do echo $i > numbers.txt;python DoMyScript.py & done