How many visitors can my server handle at one time?

I have an old server with this spec: Processor #1 [COLOR=rgb(0, 102, 51)]Genuine Intel Intel(R) Pentium(R) CPU G6950 @ 2.80GHz Speed 2799.874 MHz Cache 3072 KB Processor #2 [COLOR=rgb(0, 102, 51)]Genuine Intel Intel(R) Pentium(R) CPU G6950 @ 2.80GHz [COLOR=rgb(0, 102, 51)]Speed 2799.874 MHz Cache 3072 KB CentOS 64 Bit (Latest Stable) cPanel/WHM 2 GB ECC DDR3 2 X 250 GB Enterprise Grade SATA II 3200 GB Transfer (100 Mbps Uplink) This server only has 5 websites sitting on it with very very low traffic (less than 10Gb/month used in total). My client wants to start a "Grab your gold pendant" game. The client will inform its members (more than 10 millions members) in advance that at 2:00pm EXACTLY on, for example, November 30, 2018, people can fill up and submit a simple form at the website asking for free gold pendants. The game will only be open for about 10 minutes. It is expected that when the exact timing is reached, thousands of members will access the website at the same time to submit the form. QUESTIONS: (1) With the above spec, how many members can the server handle in 10 minutes before the site is jammed? I only need a VERY ROUGH ESTIMATION.... 50, 100, 500, 1000, 5000? (2) Is there any setting in WHM which I should temporarily employ for the sudden surge? (3) Any feature in cPanel which I should consider monitoring to see if the website is over crowded? (4) As shown in the spec, the server has 100 Mbps Uplink. How much will this help compare to 10 Mbps Uplink? Thanks.