Practical user agent implementations have limits on the number and
size of cookies that they can store. General-use user agents SHOULD
provide each of the following minimum capabilities:
o At least 4096 bytes per cookie (as measured by the sum of the
length of the cookie's name, value, and attributes).
o At least 50 cookies per domain.
o At least 3000 cookies total.
RFC6265: HTTP State Management Mechanism
http://tools.ietf.org/html/rfc6265这里说的是SHOULD，因此你不能断定某些脑残浏览器会怎么定义自己的策略。不过按照这个标准执行不会错，忽略那些浏览器就好。