If I understand the question correctly, yes, you can deploy multiple MR42 APs to support 300 users in a large room. There's much more to consider however, so planning is important. But if it's typical users, running typical applications, with typical bandwidth requirements, and with typical mix of client devices, then if you have a large room with 300 users, you would likely be pretty well served by 4 APs roughly 20% equidistant from each corner of the room, perhaps a 5th AP in the center. You might also consider MR52 (or MR53 if you have multigigabit switches) for the highest performance with a 4x4:4 AP (the MR42 is 3x3:3). a 4x4 AP will outperform a 3x3 AP especially when there's higher density and/or more noise, plus 4x4:4 will maximize MU-MIMO benefits moreso than with a 3x3:3 AP.
There are a lot of variables so a proper site survey is always the best first step, as well as considering the types of client devices and their capabilities, the applications they'll be using, and the RF environment. And if this is outdoors or very high density, there also a simple guide here for high density (stadium-style) deployments: https://meraki.cisco.com/lib/pdf/meraki_high-density_solution_guide.pdf
Yes, you can go to the Firewall and Traffic Shaping page under the Wireless menu to create various L7 firewall rules to block specific types of traffic like P2P, Social, etc and you can also create traffic shaping rules there, in case you don't want to completely block certain traffic, but throttle it. The APs by themselves can provide adult site content filtering right at the AP. More on that here: https://documentation.meraki.com/MR/Firewall_and_Traffic_Shaping/Adult_Content_Filtering_Overview And for full content filtering capabilities (plus all other security features like firewall, IPS, AMP, VPN, etc) deploy an MX appliance. More on that here: https://documentation.meraki.com/MX-Z/Content_Filtering_and_Threat_Protection/Content_Filtering
Remember you can always ask your Meraki (or Meraki Partner) sales team for a free trial of a few APs. You might also be eligible for a free AP by attending a Meraki Webinar. More info here: https://meraki.cisco.com/lp/free-demo
Let me ask you a question.
How much throughput do you want each user to be able to have?
Are you clients modern, with 802.11ac radios, or older 802.11n? And do all your clients have 5Ghz support?
We recently had a meeting in one of our halls that was able to serve 150 clients using 2 Mr-42's most clients were using the 5GHz channel to take an online survey so no video streaming or anything like that.