Two. If you have previously done all loyalty missions, you'll only need to do Legion's loyalty before the Collectors board your ship. Missions are anything that results in a Cerberus report screen. Landing on hub worlds and SB's base doesn't count as such.
Depends a bit on your progress actually. If you haven't activated/spoken to Legion and have done all other loyalty missions, you might have time to do 0 missions/assingments after Collector abduct the crew. Dogmatix314 16:48, October 16, 2011 (UTC)
^ A bit of clarification: if you want to save the crew, you ALWAYS have zero missions after abduction. In between abduction and IFF, however, you can have one OR two missions. One mission if you activate Legion AND talk to him after the initial cutscene is over; two missions if you activate him, walk away and do mission, THEN come back to talk to him - he'll give you a possibility to clean out the heretic base. You can either do that or do something else. Best course of action is to do everything before heading to the IFF (except Tali's trial), then do the IFF, then Tali's trial (bring Legion!), then Legion's loyalty. Why did I write this all again?! It's in my Suicide Mission guide! :D Land Raider 17:30, October 16, 2011 (UTC)
Hmm.. I mixed up my words, sorry :D .. Meant to say after IFF mission and before abduction like you corrected. Dogmatix314 18:03, October 16, 2011 (UTC)