Difficult to give advice, you probably should have enough funds to not worry about losing what you fly. It’s more a feeling comfortable losing than hard facts, especially as you can always sell a few PLEX if you really get broke.
There are also different types of ships, your PvP ships are like ammo, but your highsec mission runner or haulers can be lost too, but much less likely. Also they are usually flown in “prey” mode, with extra caution.
For example I’m flying and losing Hecates a lot, I need about 20 of those per month, often less. Hence with ~80M including fits, this makes 1.6B I need to sustain each month. I also spend about 500M on tags to keep my sec status above -2 if necessary, sums up to about 2B expenses.
2B per month on average I can easily make from PvP loot, trading, and the regular events without begging to my bank char.
I aim to have liquid isk equivalent to 2x the value of the most expensive asset I ever actually risk losing. So if I have a 10bil dread I like to drop on people, I keep 20bil on hand. The rest goes into PLEX which is ready for big sales on extractors or something. There isn’t much reason to hold onto huge amounts of ISK.
About 1/4 of my ‘total wealth’ is ISK. The rest is ships, plus loads of modules, ammo, etc. Most of my ships I have more than one of…if I include ships owned by multiple chars. I think the real factor is that my ISK wealth alone is a lot larger than the cost of my most expensive ship ( a 1.3bn Nightmare )…so I can ‘afford to lose’ any of my ships and have a new one right away.
As in real life, it is smart to keep a certain amount if liquidity available, but keeping too much available is the same thing as throwing money away.
I keep a number of savings reserves for various projects I work on. My general emergency fund has enough in it to buy whatever I need to start making money again if I get totally wiped out. Other reserves are sized based on whatever the needs of that project are. I’ll generally reduce the sum of all my reserves by a bit if I have multiple savings reserves at the same time.
My active wallets just kind of have whatever they have in them and then buy stuff to store value; My isk will only ever decrease in value while my jump freighters and marauder hulls go up and down, sometimes WAY up. My rare skins pretty much only go up. I don’t really like trading in commodities like alpha injectors, skill extractors, plex, etc as they don’t have the ability to be converted into wealth unless they are sold and if prices are down, there will be no wealth generated. (skins are the same way but they usually never go down beyond their first couple months of release) storing it in ships or deadspace modules provides and opportunity to use those items to make money if I had to, though only my jump freighters I keep as a wealth store have ever been needed to be used like that.