I believe somebody's mapped out the Archives that would be neccessary to get to the GDS HC. I'll have to see if I can find them. Off the top of my head, I'd say around 7.

Things get really messy if you're talking about continuing the Deluxe HC's through the Zero-Hour reboot. You'd have to figure out what all would be included first. To get to the end of Levitz's 2nd run (issue #63 of Volume 3) would take 3 or 4, depending on how big they were and what you decided to include outside of the main Legion title. There's a pretty recent thread where someone mapped those out. After that, you've got 61 issues of Volume 4 LoS-H and 5 annuals prior to the reboot, 18 issues of it's sister series Legionnaries, and 23 issues of Valor, and the 5 issue Timber Wolf mini-series. If you just limited the HC's to the Volume 4 Legion title, you'd probably be looking at another 3 or 4 Deluxe HC's to cover from the end of Levitz's 2nd run in '89 to the Zero Hour reboot in '94.