Terms of insurance policies can vary but generally slashed tires would be covered by your comprehensive insurance, no matter how many tires were slashed. Unless your policy specifically states that they will only cover slashed tires if all 4 were damaged then you should be able to make a claim if only 1 or 2 tires were slashed by a vandal. We have never heard of a 4 tire requirement and/or exclusion on an auto policy contract.
However, if only a couple of tires were slashed you may find that it is not worth making a claim with your car insurance carrier. For example if you have a SUV and the tires are $175 each and 2 have been slashed then to replace them it would be $350. If you have a comprehensive deductible of $250 than instead of basically making a claim for $100 it may be better to just pay that out personally and not make a claim. Though some insurers will not (or cannot depending upon state laws) raise rates due to Comprehensive claims some can.
So while insurance carriers should see your tires being slashed as an act of vandalism, which is covered under Comprehensive coverages the cost of replacing your tires may be less than your deductible or only a bit above and thus make it better to forego a claim and pay the replacement tires yourself out of pocket. If you get only 2 tires slashed that is all insurance will pay for so if you want to replace all 4 so that they match and have the same wear on them you will not be able to claim for the 2 tires that were not damaged.
Also keep in mind that you normally cannot receive betterment due to an auto insurance claim. The intention of the insurance policy is to return things to the way they were before the loss or damage occurred. Sometimes, when items are repaired or replaced the policyholder will end up with something that is better than it was before the loss. This is defined as betterment.
To return your vehicle to the way it was before your tires were slashed would technically mean replacing the tires with ones that had the same mileage and wear and tear on them. That would be indemnity for the loss, returning the vehicle to its state before the damage occurred but it is doubtful that many policyholders would want used tires as replacements.
To not end up better than before this damage was done to your tires, the insurance company will likely take into consideration the mileage as well as the wear and tear of the tires that were slashed and may require you to pay the difference between what they were worth and the price of new tires. So you get the depreciated value of the tire instead of the cost of new tires many times with insurance providers when you make this type of claim.