You should definitely mention that you attended conferences. How much detail to include depends on how much space you have left after you included all of the more important information on your CV. I cannot tell what is more or less important. You have to figure it out yourself. Here is what I usually do in such situations:
- Make a CV listing everything you have that is even slightly relevant to what you are applying for. If you went over the page limit proceed to step 2.
- Divide your CV into absolutely necessary/required info and 'other stuff'.
- Rank the 'other stuff' from very important to barely relevant on some continuous scale, say 1 to 10.
- Delete the least relevant info until you reach the page limit of your CV.
You can apply the same procedure to your conference list. I would say it is important, but not 100% of it if you don't have enough room. Rank the conferences you attended from the most prestigious/impressive to the least impressive. Start listing them from the most prestigious until you hit the page limit. Leave one sentence to mention the conferences you omitted, generalizing them in some way to underline their relevance: "... and other (number) conferences on topics related to molecular biology.