How many true-bypass pedals is too many?
Guitar players have observed that a bunch of true-bypass pedals will cause degradation of their "all bypassed" tone, which kind of defeats the purpose of a true-bypass switch, which is to avoid degrading the uneffected tone of your guitar. It's true. It depends upon a few factors, but you'll start running into problems at around four or five true-bypass pedals.
The problem is capacitance. You know that a longer cable from guitar to amp will cut treble because it adds capacitance. (The added capacitance also shifts the resonant frequency of your pickups to a lower range. Most people perceive that as part of the treble loss.)
Cable is rated as having a certain amount of capacitance per unit length. Most cable used for guitar is in the range of about 16 to 48 pf per foot. What most people don't account for is that the plugs add capacitance, as do the jacks, switches and wiring inside the stompbox. You may only have a few inches of cable from one box to the next, but all that other stray capacitance adds up. You get the electrical equivalent of a couple feet (give or take a bit) of added cable when you daisy-chain two effects stompboxes.
So take four true-bypass stomboxes and chain them together. Leave them all turned off. You've just put the electrical equivalent of almost ten feet of extra cable between your guitar and amp. Can you hear that as treble loss? Sure you can. Get two instrument cables of reasonable length. Same brand, same model, just differing in length by ten feat. Compare them. You'll hear a bit of treble loss with the longer cable.
Now you have to ask: is this a big deal? It depends upon how many stompboxes you use. With up to three or four stompboxes, all true-bypass, the change will be audible but probably not objectionable. A lot of the things you can hear doing A/B comparisons in a quiet room just don't matter once you get into a room full of noisy patrons with the band playing full-tilt. This isn't rocket science, after all...
A lot of folks have ten or more pedals on their board. If these are all true bypass, I can almost guarantee that the treble loss (and resonant peak shift) will be painful. With all the pedals disengaged, it's like adding another twenty or more feet of length to your guitar cable.
But the treble loss is only half the story. What happens when you engage an effect is also important. Every effect has a low-impedance output. When you engage a true-bypass effect, it's as if you're also adding a buffer at point in your signal chain. So all of a sudden the guitar is only driving the cable and stray capacitance up to that first engaged pedal. The total capacitance apparent to the guitar drops, and it sounds brighter. If you have a short cable from guitar to board and a long cable from board to amp, that brightness shift can be very noticeable.
So what do you do? You can add a buffer to your board. Any buffered pedal (Boss, for example) will do, or you can spend a hundred bucks or more on a box that's designed as a buffer. In the latter case you'll probably get better signal quality, but remember that the improvement probably won't be noticed in a live music venue. The most common place to put the buffer is at the head of your signal chain, but after fuzz and sometimes wah. If you have only a few pedals, you can also put the buffer at the end of your chain to isolate the capacitance of the cable from board to amp.
I personally don't like to use a buffer. It's just one more thing to go wrong. I set up my board so that I don't really need a buffer. Between only having three pedals and using a short cable from board to amp (to minimize the treble shift when I engage a pedal) I get the best of both worlds. When all the pedals are disengaged my guitar goes "straight" into the amp with no active electronics in the way. And when I engage a pedal, my guitar's basic tone doesn't noticeably change.