depends if the fort has soldiers in it and what soldiers .... the lvl of your range attack
you need 200 trebuchets for a lvl 7 fort
to be sure you can use the battle simulator
i lost a siege a year ago against a fortress 7 with 500 trebuckets, my problem was that i had low moral. If you have good moral 105-110 go with 300 to 400 trebuckets accompanied by some 50 to 75k guardians. try using the calculator, while doing so try not to underestimate the enemy fortification levels
. then at the end check your end moral to see if its above 50. lots of times the calculator says you will win, at times with 30 and 36 end moral, dont be fooled, you might just not be lucky. Hope that helps