Do monsterous calvary get monstrous support attacks

[Kaiserdean]

The way the rules are worded, the monstrous cavalry get the bonuses of monstrous infantry rules, but it does not specify the monstrous support rule (which is a separate ability listed in the monstrous infantry entry.)

Do monstrous cav only get one support attack? Most everyone in my LGS think it's three support attacks but we can't find anything to rule for our cause.

[Chronowraith]

In short, no. Only the rider of the monstrous cavalry get supporting attacks.

As you stated, Monstrous Support is not listed under Monstrous Cavalry. According to the BRB, Monstrous Cavalry follow all the rules for Cavalry with the addition of Stomp, Monstrous Ranks, and the use of the mount's W characteristic. That means only the riders get supporting attacks.

I do believe this is even the same for Mournfang Cavalry despite Ogres being Monstrous Infantry when unmounted.

[Lord Sigmar]

Do monstrous cav only get one support attack? Most everyone in my LGS think it's three support attacks but we can't find anything to rule for our cause.

Yeah in the brb only monstrous infantry get the 3 attack support (monstrous support), however the faq fixes this

Page 83 – Monstrous Cavalry
Add “MONSTROUS SUPPORT
Furthermore, the rider of a monstrous cavalry model can make
as many supporting attacks as are on its profile, up to a
maximum of three.” to the end of the paragraph.