As far as OSPF, I know that Cisco recommends that you should not have an OSPF area with more than 90-100 routers. Is there such restriction or recommendation for ISIS areas or levels?
In theory, just like for OSPF, there is no limit. There will be a practical limit, based on the routing protocol traffic generated, bandwidth, and router resources.
Some people have networks that work with 1000 routers for either OSPF or IS-IS, but that would be rare, and it is probably a network that needs to be redesigned. I actually worked on one, and it took over an hour to converge. Eventually, it was broken up into ASes, using BGP between the ASes, and OSPF withing the ASes.
In practical terms, you will probably see no more than a couple of dozen routers in any OSPF or IS-IS area. More than that would probably be symptomatic of a network that needs to be redesigned.
All the areas, except the backbone are on the same level, so you can have an almost unlimited number of areas at the same level, but they will be separate areas.