Regarding projectiles, not many. Since they will take take damage and die, get knocked into oblivion, possibly arch over the projectile and miss, the fact that they are part of Olimars arsinal means that he can't spam them and double as a threat. In most cases like Link, Diddy, Samus, Mario and a few other, Olimar does not have an "unlimited" supply of spamable projectiles, as well as be able to use them as a solid approach like the above characters mentioned. (not counting the coveted Purple)
As far as blocking actual attacks in an Olimar Attack Vs Opponent Attack, I'd say virtually none. I think most characters either out prioritize him, or they have a bigger and better disjoint. I do wonder what would happen in an Olimar Uair vs a Link Dair. I figure the first would hit the Pikmin and the second would hit Olimar, not sure though.
This isn't to say that Olimar looses on priority to every move, but I do think he looses to more than his fair share, along with no penalty to the opponent during the clash.