Will there be less cops if I play solo or with only AI?

According to an extremely lengthy guide on Steam, the number of players does impact the number of enemies that can spawn in a heist. Difficulty is another key factor.

The following is the limit of how many enemies can be on the map at a time:

The guide also notes that:

Criminals who are being tased, downed or in custody do not count. When playing solo with bots, subtract 1 from the number of criminals. When the number of criminals is less than 1, it is forced to 1 for these calculations. Once the number of non-retiring recon and assault enemies exceeds this threshold, the assault task does not spawn more units (the recon task itself does not consider this limit to determine whether to spawn units). The limit is chosen at the start of each assault.

A group of 4 players at 100% diff on One Down will have to deal with at least 84 assault enemies (up to 88 possible if a group of 5 spawned at 83), in addition to reinforce enemies and scripted enemies.

Now, the maximum number of enemies in an Assault Wave is the following:

Furthermore, the guide points out:

Specials count as one spawn just like ordinary enemies. If all spawns are used up during the assault wave, enter Fade. The system is the similar to before; but the pool size is adjusted every 2 s (depending on all criminals in full control of their character, as before). The other tasks and scripted enemies do not drain from this pool. Killing an enemy does not affect this number other than potentially making room for more assault groups (it is the process of spawning, not killing, which drains from the pool).

The guide goes into great detail about how spawning works in this game. One thing to also considered is that scripted enemies do not count towards these values as noted in the guide. These enemies include gangsters, snipers, and cops (the basic ones).

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As far as I am aware, if you are doing a stealth mission, so long as you maintain stealth, the only thing that impacts the number of enemies is the difficulty you are on. The higher it is, the more guards there are patrolling areas that may not have a guard on a lower difficulty. The only way more enemies spawns in stealth is if you have 4 "uncool" or dead guards. As the guide states:

On some levels, a new guard will spawn 145/120/120/80/45 seconds (depending on difficulty, where 45 s is for MH+) after 4 enemies are not cool, i.e. either uncool or dead (as dead people certainly are not cool)