1) can we change the email address associated with our account or its not possible after you sign up?
Of course it's possible.
AFAIK, this process takes one week and your account will be locked during this period. You can refer to this quote from admin:
As an extra protection against any possible social engineering attacks, whenever
* the administration changes an account's email address from its current value, the following process occurs:
- The change is queued.
- It is listed in
seclog.php.
- The old email receives a warning.
-
After 7 days, the change goes through and another seclog.php entry is added.
The account stays locked throughout all of this.
Hopefully it will be essentially unheard of, but if an account is going to be incorrectly transferred, everyone who knows about the incorrect change should noisily post all of the evidence they have so that we can at least put the change on hold and re-review the evidence.
* Admins can act outside of procedure and bypass the queue if necessary, but hardly ever will.Note that in case you lose access to your account, you can restore it only to the original email used for registration even you had change it later.
2) is there any rough guess of how many users are there on bitcointalk.org
The number of registered accounts doesn't refer to the number of members.
In brief, there is an unknown number of users using an unknown number of alts.