It is pot luck and impossible to predict exactly how many guys will book and turn up, but it hasn't been overly busy this year when I've attended the new Phoenix venue...my last party was 7 guys and 5 girls.
Even with a full party ie. the max 2:1 ratio, there's still action to be had as most girls can service 2 guys at a time and as guys start popping and taking breaks there's always room for some 1-2-1 action.
A lot of guys prefer the busier parties as there's often more atmosphere and banter, but I can appreciate for a first-timer feeling his way, a more chilled party might be more desirable.
I remember overhearing Em say the 4-6pm party is usually the least attended of the day. Nevertheless, my advice is to go with a line-up that appeals, at a time when it suits you and you'd be most up for it...enjoy!!