The distance that you can connect to your WiFi hotspot will be determined by the signal level broadcast by your mobile phone hotspot. As transmit power has a direct impact upon battery life of the device that is hosting the hotspot, this broadcast signal level, so as to maximise battery life of the hotspot, will typically be a much lower than that broadcast by your WiFi Router.
As to your second question, you can calculate the maximum number of potential client devices that the hotspot can support by looking at both the IPv4 Network Address of the WiFi hotspot - and the subnet mask. The subnet mask will tell you number of addresses hosted by the network subnet - from which you must delete two (one being the network address, the other being the corresponding broadcast address).
If these terms mean nothing to you, you may need to learn at little about IPv4 network addressing to find your answer.
However, whilst this is not the place to provide an in-depth explanation of IPv4 Networking - using an iPhone as an example, the WiFi hotspot subnet mask is 255.255.255.240. This means that the subnet supports a total of 16 IP Addresses, the first being the network address and the last being the broadcast address. This leaves 14 usable IP addresses for use by client devices. Although the subnet supports 14 clients, the hotspot may not allow all 14 available addresses to be used.
I hope this information and insight proves to be helpful to you.
