@phanlawi is the current 100Mb bandwidth on a 100Mb bearer or a 1Gb bearer? If it is on a 1Gb bearer and the ISP you use is able to manage simple soft changes then you have two options:
1) calculate the max number of concurrent video streams going externally and multiply by the bandwidth used. Upgrade to this amount, it might be more than you need though.
2) add 50Mb, change the WAN port settings on the MX100 to reflect the change and monitor the improvements, if some but not enough then add again. This will ensure you don't overbuy.
We use MXs for our SD-WAN and I can confirm that the MX100s will pass at least 500Mb of traffic from ~200 devices without overloading so you shouldn't need a hardware upgrade unless you plan to go much higher. The 750Mb limit mentioned by @CptnCrnch is a hard limit.
If my answer solves your problem please click Accept as Solution so others can benefit from it.