You can summon as manu times as you have the actions for and the resources usualy the right card (and having the appropriate model of course).
Some summoning actions may have other restrctions but these will be noted with the action on the cards. It does not need any additional soul stones unless specified by the action.
For example Kirai has the restriction of only being able to summon resurrectionist spirit minions and must be summoned in base contact witha friendly model within range.
Kirai dosnt have any errata to her and summoning isnt particularly broken or unbalanced. To defeat summoning you have to build your lists appropriatly (which may be difficult if your opponents only have limited collections) and learn to play to thier lists strengths and take advantage of your lists weaknesses.
The best way to beat summoning is to put pressure on the summoner and drain the crews resources, summoning often needs high cards which leaves the rest of the crew without them for other actions and for dules. If a crew needs cards for defence then it wont have them for summoning and vice versa.
Also make sure you are playing with schemes and stratagies (malifaux dosnt really work if your not using them its not a game designed for streight up fights), some of these are unfavourable to summoning as they give up vital VP. Remember its possible to be tabled and still win if you are playing to your schemes and stratagies.
I would suggest getting your opponents to read tactics for thier crews or even swapping crews with them for a game so they can see how summoning crews work.
Ikiryo can be summoned and activated multiple times a turn but this is very resource intensive (which again leaves less for your crew to function) and requiers your opponents to target the correct models to activate malevolence. Your opponents will need to learn how to identify the best targets and determine thier activation order to best combat the ability, but this is something that will come the more they play. The more games you play the better you will get.