BPMN is very popular notation for process modeling and it's theory is based on task sequences and work flows between actors. So it's seems that it's necessary condition to declare at least two lane (Actor) for model. But sometimes i find independent processes that only one actor has been engaged. Is it valid to draw such BPM with just one lane?
The purpose of a swimlane diagram is specifically to show the interaction between different functional capacities or organizational roles.
Can BPM be created with just one lane?
The diagram would be valid (as there is no rule against this) though uninteresting (as a swimlane diagram) because it is essentially degenerate.
A swimlane diagram with only one swimlane is probably showing a business process that is either insufficiently decomposed, or not shown in sufficiently rich context.
As far as decomposition, since the point of the swimlane diagram is to show roles and how their responsibilities interact, a swimlane diagram showing just one role (e.g. the whole company) is really not telling us much about who does what. Chances are pretty good that you can decompose the single swimlane into more detail to identify separate interacting departments in the company.
Regarding context, if there are any inputs or outputs of the process, then actors for these are the other swimlanes, so with them you'll then have two or three swimlanes. A process without inputs & outputs is arguably lacking in significance.
A one-swimlane diagram is more like a flowchart, which identifies steps and flow, but doesn't identify the roles. Roles are a relative concept — they are relative to each other, e.g. customer﹘supplier, sender﹘receiver, etc.. So, if you have a role, then almost by definition, you have another role.
That being said, a one swimlane diagram can sometimes be a natural start to a more complete diagram, so, most tooling would allow you to create and save such a diagram.