We are at 994 Skins right now. The next skins that are gonna be released are the True Damage Skins, we have 6 of them. Which means the 1000th Skin is gonna be a True Damage Skin.
I thought Riot would maybe make something cool (Like another Riot Skin) for the 1000th Skin, but it doesn't look like it... except if you see Prestige True Damage Qiyana as something special. Kinda upset now, maybe they forgot or they don't think it's a special occasion?
EDIT: I've made an error, there are 994 Skins in the game WITH the True Damage Skins, which means the 995th Skin will be Pulsefire Thresh. So we still got a chance for a nice 1000th Skin! (Thanks to 'Scuttle' for pointing my error out)
EDIT 2: **Prestige** Pulsefire Thresh and Victorious Aatrox will be the 996th and 997th Skin! (I didn't know that Pulsefire Thresh would get a Prestige Edition, seems a bit odd to me anyways).