If a character has 3 natural weapons (2 claws and a tail), and has the feats Improved Multiattack (eliminates penalty to attack rolls for secondary attacks, but still uses 1/2 Strength damage bonus) and Improved Rapidstrike (can make up to 4 additional attacks with natural weapon pairs with a cumulative -5 penalty each), how many total attacks would he potentially be able to pull off in a single full-round action?
Five: Three with one Claw, once with the other, once with the Tail
Improved Rapidstrike means that you get two extra attacks with one of your claws, and you still get your other claw attack and the tail attack (which are not affected by Rapidstrike), up to a maximum of five attacks.
Your attack routine, assuming +15 BAB (minimum for Imp. Rapidstrike) and 1d6 damage on each natural weapon, would be as follows:
+15/+11/+6 Claw (1d6+Str), +15 Claw (1d6+Str), +15 Tail (1d6+½Str)
The extra attacks granted by Improved Rapidstrike are not secondary attacks so they are not affected by Improved Multiattack. A pair of natural weapons can both be primary, so the second Claw attack isn’t affected either. Imp. Multiattack does affect the Tail attack, which is secondary; without it it would be at −5 or −2 with Multiattack.
Personally, without more secondary attacks, I wouldn’t spend the feats on Multiattack and Improved Multiattack. Even with a few more secondaries, I usually find Multiattack to be enough.
He vould be able to make a grand total of 7 attacks. Five of those would be iterative attacks from his main weapon, with the usual cumulative penalties, while two would be his other two natural attacks, at his highest attack bonus due to Improved Multiattack.
This is without factoring in bonus attacks from haste or similar effects.