I am learning about cookies, and I wonder about browser support when writing web applications that rely on cookies to store state.
For each domain/web site, how many cookies may be sent to a browser, and of what size?
If multiple cookies are sent and stored, does that affect performance?
Firstly, I suggest you don't worry about this issue. There is AMPLE room to serialize tons of identifiers to.
Secondly it's not stored by web-server but by web-domain -- e.g., www.google.com and not the 100's of different physical servers that serve the Google domain.
Thirdly if you do have to worry know that their are two possible cookie headers. The sizes of these cookie headers are determined by the browser software limits.
Design Discussion
What you don't want to use the cookie header for is sending details about a client's session. E.g., don't try to stuff the email a client is typing into a cookie if you are building an email front-end. Instead you would send the client a cookie that represents his identity+session: you store all sessions data against this identity. You can store tens of identifiers (4-16 bytes) per cookie header and no one needs more than say 4 of these. The cookies data (as an integer) tends to be encoded to base64 which increases the byte-count.
Performance
Your browser sends a plethora of headers to a web-server. The cookie is just another 100-1000 bytes (mostly closer to 100). At both extremes it takes only a fraction of time to send these to the web-server -- when placed into context of course. You should keep in mind that the web is built on text based protocols.
Different browswers have different size limites on cookies. Here is the information for IE. Here is a page that lists several browsers.
Cookies are not saved on a server basis but on a domain basis (a server may host many domains or the opposite a server farm may be serving a single domain).
In general, I would avoid saving lots of information in cookies, as the data gets sent to and from the browser on every request. As you suggest in your question, this can have a effect on performance.
Usually one stores small amounts of data in the cookie, mostly used to identify the user/session so more data can be picked up from a database or another resource local to the web server.
If you're programming a web site, it's a good idea not to store too much in a cookie, because that cookie gets send to the server every time the user requests a page from your site. A far better solution is to just store a unique id in the cookie, and let the server pull up the required information from a database or file store based on that unique id. Unfortunately that solution leads to people worrying about what you're tracking about them, so you might want to have a "cookie policy" expressed somewhere on your site talking about why you're placing a cookie on their browser and what you do and don't track about them.
4 KB per cookie. 20 cookies per domain in Internet Explorer 6. This means the common denominator is 80 KB of data, split across 20 chunks (clumsy at best).
Generally it's recommended to preserve state on the server, and use cookies only for session tracking. They're sent along with every request, and you really don't want dozens of KB of data crossing the wire on every server call.
If you do want to keep state on the client, and you can use JavaScript to do it, there are options. The PersistJS library implements all of them (as a convenient key/value store). Dojo storage is another well known cross-browser local storage library.
A short summary of storage options:
So, generally it depends on your use case: