|I can't see you being able to connect a second router that's on a different subnet wirelessly.|
This is easy enough using a network cable to connect the two because you can connect from a LAN port on the upstream router to the "WAN" (internet/external) port of the downstream of the downstream router.
However, with wireless, you would need to put one of the two in "bridge" mode. Bridging is not routing. One uses a bridge to extend a single subnet/network, not add a second, different subnet/network. Bridges don't do routing and I suspect (but I could be wrong, I've never tried this) that a SOHO router will not route while in bridge mode.
So you have two choices as I see it:
1) Put the downstream router on the same subnet as the upstream router and connect it in bridge mode wirelessly.
2) Connect the two router's with a network cable and configure the downstream router with a different subnet.
If you have to go with option two you will want to click on my name above in this response and read my “how-to” guide titled, “Add a second Router to your LAN”
You're going to want to use the separate subnet scenario wherein you interconnect the two routers "LAN port to WAN port"
I know you said you can't connect with a cable but you should take a serious look at pulling one. If you can't, then you'll have to extend (bridge) the present internet subnet to the second router and connect it (second router) in bridge mode.
It can support up to 255 users or devices (addresses) concurrently.
I've found with SOHO equipment you can get about 10 to 15 wireless clients concurrently connected before they reach saturation point. Once they reach saturation point (ie: as many concurrent connections as they can easily handle) they begin dropping connected clients when a new connection request comes along. You end up in a round robin situation with nobody being able to be stay connected for any reasonable lenght of time.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.