Upvote:5
Finland has one of the highest success rates for multiple-entry visas (about 80%).
If you envision multiple visits and you have a good premise to support multiple visits, then you should apply for a validity of 1 year and use the Schengen 90/180 rule to govern your actual visits. This advice would be true even for your first application.
There is no risk in asking for a year. If they do not think you qualify for a year, they will downsize the validity to something they are comfortable with. If they grant you a year, then you have saved.
See the META thread: Unanswered or dubious answers on Schengen visa questions for some background on this question. And remember that your premise is key-critical.
For your other questions...
And what should I mention in my cover letter as my sole intention is of tourism?
Get real. The overwhelming consensus is that you tell them the truth. Explain your intentions with natural language and use a translator if needs be to assure that it is clear and crisp.
Also, for multiple entry do I need to have insurance of complete duration of one year for which I am applying or is it enough if I have insurance for the ten days of my first visit?
Get coverage for your initial visit. When the border officials see a multiple visit visa they may challenge you for insurance so be prepared for it.