Hi there,
Power supply lines are like a river - looks quiet but it's very lively and noisy, very noisy and full of waves close up.
Good practice is (at least) one decoupling cap per IC (and more if they have a lot of IO pins), as close to the pins as possible, so they aren't disturbed by adjacent devices and likewise don't disturb either.
Another orientating very rough rule of thumb is that generally 100pF (or less if you do the calculations) for high speed switching devices, 100nF for 40xx type ICs, 1uF for insurance against disturbing other devices on the PSN, and maybe even 10uF for noisy things like 555s.
Apparently, it's good practice (or it isn't, depending on what you read) to parallel capacitors of the same value, can't remember why right now but it makes a lot of sense. Also, apparently there's no harm in scaling caps that are paralleled so long as they have a ratio of 10:1, e.g. 1uF:100nF:1000pF.
Looking at photos of circa 1960's circuits there are few caps in sight, looking at current circuit boards you'll see they are filled to the brim with minute SMD caps under and around the microprocessors, if that helps.
I personally wouldn't have any ICs sharing 1 decoupling capacitor as that is no longer decoupling, that's what the filter caps at the circuit input are for. And OTT personal opinion - I try to avoid aluminium electrolytics like the plague unless it can't be helped due to capacitance needs/PCB space/budget, and ceramic are a touch wobbly too where temperature and stability are concerned.
- - - Updated - - -
Hi again, took a moment to remember how to attach pdfs/docs, etc. I have a low frequency brain.
Here's a folder of useful stuff about decoupling and bypassing, if anyone who's interested can bear to read through it - well worth the time though if you skip the parts that are irrelevant to your needs.
Also, this web has some interesting stuff about capacitors:
**broken link removed**
Specifically:
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View attachment Capacitor Bypassing and Decoupling info.rar
Never leave home without a handful of passive components in your pocket, especially capacitors.