1. Does monowall support many open connections?
How many is "many"? I'd say yes.
I've tried killing m0n0wall by starting like 30+ well seeded torrents and upping the connection limits on the clients, m0n0wall didn't break a sweat (this on a Alix).
2. Major difference between mono and pfsense?
Someone else can probably answer
this.
3. I am looking for QoS with IP or bandwidth limiter on IP does mono support this feature?
Yes, as you could have found about
here.
and any suggestions on what spec I should use will be great.
How would anyone be able to answer that without knowing anything about your intended use?