The complete error message should reveal the solution:
line 2: LIMIT: command not found
the word not recognized as a command is LIMIT. Indeed when you assign a variable, you should pay attention to the extra blank spaces: actually they must not be there, that is the equal sign should be immediately after the variable name without spaces before and after:
Regarding the input from the user, try the read
read -p "How many terminal sessions? " ans
for ((a = 1 ; a <= ans; a++))
gnome-terminal -e "ssh -X firstname.lastname@example.org"
Hope this helps.