TO Create Workflow we have to goto Setup -> Create -> Workflow & Approvals.
I am unable to see this "Workflow & Approvals".
I am having developer Org and I heard that this should be available in developer Org.
I tried changing some settings but no luck.

The Recycle Bin link in the sidebar lets you view and restore recently deleted records for 15 days before they are permanently deleted. Your Recycle Bin record limit is 25 times the Megabytes (MBs) in your storage. For example, if your organization has 1 GB of storage then your limit is 25 times 1000 MB or 25,000 records. If your organization reaches its Recycle Bin limit, Salesforce automatically removes the oldest records if they have been in the Recycle Bin for at least two hours.

Just want to confirm whether it will be 25,000 only or more than that.

Unable to create Master - Detail Field on Child Object.
If I am trying to create 2nd level  Master - Detail. Or making Standard Object Account as Master.

Error is not showing you cannot create 2nd level Master Detail OR you cannot make Standard Object as Master.

Below if link to same discussion:

https://developer.salesforce.com/forums/ForumsMain?id=906F00000008ybkIAA

This link provides 3 points to do to create this Master Detail relationship:
1. Create a lookup from the child object to the parent object.
2. Populate ALL the records with a valid lookup value to the parent.
3. Change the Lookup relationship to a Master - Detail relationship. This is only allowed if ALL RECORDS HAVE A VALID LOOKUP. Otherwise you'll get the error you describe.

What is 2nd point in this?

I have Customer who can have multiple accounts. In Customer there should be field which tell no. of accounts that Customer is having (through Roll Up Summary Field).

The Recycle Bin link in the sidebar lets you view and restore recently deleted records for 15 days before they are permanently deleted. Your Recycle Bin record limit is 25 times the Megabytes (MBs) in your storage. For example, if your organization has 1 GB of storage then your limit is 25 times 1000 MB or 25,000 records. If your organization reaches its Recycle Bin limit, Salesforce automatically removes the oldest records if they have been in the Recycle Bin for at least two hours.

Just want to confirm whether it will be 25,000 only or more than that.