You can run as many fundraising campaigns as you want with the Charitable WordPress plugin.
You can choose to donate a fixed amount, enter your own amount, or do both. The donation form can be placed on your pages, posts, sidebars, and modal popups.
If you want to use payment gateways like Stripe, Authorize.net, etc. and newsletter signup forms, you have to upgrade to the Essentials plan for $119 a year.
Their $249/year Growth plan offers powerful features like recurring donations, crowdfunding, peer-to-peer fundraising, and more.