Maybe around 6-10 parlay, I think that would be fun as it's not easy to hit that, or even lower to make the competition quite achievable.
Do they allow 6 betslip in parley? I think no. BetBTC allows 5 per betslip as far as I'm concerned. It's not that hard to win a parley with such low number of matches/events. If I'm correct, I have won twice out of 5/6 in BETBTC. Anyway, increasing the number of matches in parley would be great I guess.
