Johnwill is correct on all points.
To go further, provided you have a router and several hubs, you can connect 254 computers to the cable connection. As for slowing down, your bandwith will be shared only if someone is downloading at the same time you are.
Example: Let's say someone else on your network is downloading a large (or small) file at the same time you are. Your bandwidth will be split 50-50 with that other computer; however, if they are just browsing, and you are too, the chances you both will click on a link to change pages at the same instant is slim. In that case (just browsing) you will not see any slow-down unless you do click on a link to change pages at the same time, which won't happen very often.
Now, let's say you do have over 200 computers on your home network and someone browsing the web on all of them. Ah then, the likelihood several others will be downloading pages at the same time is very high. At that point, your bandwith will probably divided many times and the speed will suffer greatly.