[Answer]-Python blocktrans with not working

You are doing it wrong. In template you need length template tag:

<p><b>{% blocktrans with EventsList|length as Count %}{{ Count }}</b> events detected from <b>{{ first_session_start_time }}</b> to <b>{{ last_session_end_time }}</b>.{% endblocktrans %}</p>

In templates you can not execute Python code. The template language has its own way of dealing with things.