The number of horses sold is not specific (it could be as few as 1) but but it is the intent that counts. It is something which has to be proved in court sometimes but where the person has a history or track record of selling a horse for financial gain they are deemed to be a dealer.
When looking at it in terms of having to pay tax, anything that is done for a hobby and does not result in regular or substantial financial gain would not be charged income tax. Anything more regular or substantial that would be difficult to prove as a hobby would be charged income tax. Its a fine line between the two if you have something in mind and not sure which category you fit into i'd seek the advice of a tax accountant
Thank you very much for replies. Just interested as there is some one I know who sells well over this amount of horses and is still not classed as a dealer. Im shocked that they have managed not to be investigated.
If you sell a horse for a substantial amount, you do not need to pay tax if you replace it with like.
Bearing that in mind i'm sure theres ways around things.