my stags horn sumach seems to be dying

Suckers wouldn't affect main plant, unless the main plants naturally reached the end of its life, which would explain the many suckers being produced.
Or perhaps a sucker has been weedkilled if it gone next door or out on street etc. That would get back into the main plant

I think you are right Nicky,there is probably something wrong with the main plant.This summer we have had suckers coming from next door,many feet across and into the lawn.I pondered what to do as I couldn't use a weed killer.
I just pulled them out as best I could.Then a couple of months later I realised our neighbour had dug up the tree anyway !! I have since painted weed killer on the leaves and they are now gradually dying :)
If you really love this plant I suppose you could remove the old one and save the suckers but it seems a bit of a thug if crossed !!

thankyou for your advise.,i am at a loss as it hasnt been weed killed as it is contained only in my garden,so im assuming it may have come to the end of its life.even tho it is only bout 5 ft tall,bit odd i think? im no expert with gardening ,so pls could someone advise me how to remove the main plant? thanx very much.

Just dig it out with as much of the root as possible, leaving as fewer piecs as possible. This will help the risk of the rotting roots causing problems in the future. Never dug one out, so not sure how easy it would be. May be better to wear long shirt and gloves, possible an mask and goggles as they are very finely haired and all that movement and inevitably standing up in it to dig will cause them to float around in the air. Not nice having them lodge in eyes or throat!

thankyou very much,i will do that.