For my project, I will need to connect around twelve 18650 lithium-ion batteries in parallel. Is it necessary to protect every single one of them, or are there any protection circuits that can protect and balance the block as a whole? And, if I use unprotected batteries and connect them in parallel, will the resulting circuit be safe for the usage and charging? In industrial packs of parallel 18650 batteries (like in Tesla cars), is every single battery protected as well? The batteries used for the project are standard Samsung 25R 18650 2500mAh 20A ones.
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Balancing is unnecessary among parallel-connected cells, since they will self-balance. It can be a good idea to put at fuse on each cell, so that if one cell develops an internal short, the other cells in the group won't discharge through it (possibly leading to some exciting fireworks). I believe Tesla and some others do this. However, on the majority of smaller battery packs I've seen (ebike, etc), this isn't done -- the cells are simply spot-welded together to nickel strips with no additional fusing.
Note that if you do not fuse the cells, it's especially important to never over-discharge the group -- over-discharge is one of the primary causes of internal shorts. It would be wise to build in some guard-band to your low voltage cutoff point in case some of the cells in the group have discharged faster than others (due to varied cell capacity and internal resistance).
Also note that you do NOT want to connect "protected" cells in parallel. When one cell's protection mechanism disconnects it, the current through all other cells will immediately increase, which could well exceed their specs. Protected cells are really mostly for use in single-cell scenarios.
