Heya,

define 'good'. Veterans are the most important though.

Some players are happy with 50 vets, some are with 200. I usually get my vets up to 109 and with events some higher. Experts should be up to as much as possible for better performance too, but is doable with less when your sailor has vets.

And arround 200 vets? how much experts? 300? 500?

With that amount of vets I am not sure what difference experts make. I'd put at least a few hundred on there, but that's me

It depends on the situations.  If you reach the cap for gunners, additional experts or vets are not very useful.  Similarly, if you reach the cap of repair (280/s repair), additional experts and vets for repairmans are not useful.  The only ships need a lot of vets are cv and ss (fighters in cv and all sailors in ss). 

If you are talking about ships in blizz, engineers and repairs do not do much so gunners are the sailors you want to put experts on first. 

With the event, it is easy to get to the first 40 vets by converting/rolling.  Then, the success rate decreases dramatically until around 100 vet.  Then, beyond 100 vet is more difficult.  Use your vet items only between 40-100 vet (or closer to 100). 

Also, your gain in experts each game decreases with the number of experts on your sailors.  If you are leveling your sailors, leave some rooms for expert gains (i.e., do not fill up your sailor with full experts). 

100 plus vets is good with me. I don't have the patience to roll anything more than 100. You need good luck, and lots of money to roll more than 100 vets.

ty for answers, but if i have 150-200 vets, is good option burn all experts to veterans? or how much expers i need?

As others have said, how many you need depends on preference and where you are. At low levels (DD, early CL), the benefit of high vet/expert count is not that great. At CA-BB2 level, it starts making a major difference. And then the value begins to drop as you get to higher level ships. 

At blitz levels, my recommendation is as many vets and experts as you can get on your gunners, with the other sailors not being as important. As you get to low level BBs, instead switch your focus to engineers (same rule, as many as you can get) - the higher speed and longer overheat duration will not only be your best chance for getting attack in GB (and sometimes even surviving!), but will remain relevant all the way up. As you reach higher level BBs (BB4+), the benefits of high repair will become much more apparent, and you may want to invest there. 

For the record, when deciding whether you want to go for more vets, 1 vet grants you the ability of 4 experts. So, if you're limited in funds to buy experts, any time you can convert 1/4 of your attempts or more, you will gain a net advantage by doing so. If you need more than 4 tries for each success, you're losing capability overall. At 150-200 vets for all sailors, you're in the more than adequate, but not spectacular category. I would not try for more vets unless you can replace the experts you burn.