During a mission and when exiting it, you can carry as much as you want, except for being limited to two two-handed items. The limit, which increases with time as DAvid_Spafford mentioned, only restricts how much you can take with you at the start of a mission.
When you switch items by pressing I, I think the weight and rarity are shown in a corner. If not, you can press M to open the map and then switch to the “Inventory” tab, where you can see each item in more detail. There’s also the “Field Manual” which you can find in the “Intel” tab. It has brief explanations of the most important features, in case you missed any tutorials as they popped up or want to check them again.
As you gain experience in the mode, you will learn what you can collect and where, but in case you want a little help, there’s a topic and Google Sheet here: