A different Demo account for each portfolio? I do NOT think that is what most people are doing -- at least, that's not the way I work.
Over time you'll be creating and testing 100's of portfolios. Do you really want to keep track of 100's of Demo accounts?
Also, some brokers may place a limit on the number of Demo accounts you can create. They may also place a time limit -- e.g. 1 month, 3 months -- and then the Demo account is deleted (along with all your historical data) -- which happened to me recently.
There are a few ways I can think of to limit the noise from other portfolios and strategies:
1. Choose your magic numbers wisely and then use Account History's sort facility.
2. Run a portfolio for a week and use Account History's 'Custom Period' to filter-out older strategies by specifying the date range a particularly portfolio was actively trading.
3. And there are probably scripts and indicators available that allow you to select a particular range of magic numbers for analysis. Even if you are not a programmer -- this would be easy to code and could serve as a good first project.
And there may be other ways to roughly achieve what you are looking for. Since you probably shouldn't be spending that much time in a Demo account -- after all, it's just a sandbox that only approximates real trading -- then it seems that creating a Demo account for each portfolio is way overkill.