If ya take a look at the back of the cards for many of the decorative items, you'll see that there isn't a limit listed there like the 99 limit on crops, kitchen items, basic building materials and most craftable things.
Royal decorations are the exception... There is a limit of 55 for those, combined. (Although, the back of their cards won't tell you that, lol.)
Hope that helps.
Ps. There also used to be a limit on walls, towers and gates (probably because they give RP) until TL continued to introduce new walls, etc & removed that limit a lil while back.
Pps. Some other items have a limit BELOW 99. For example, faeriebulb seeds cap at 50 and most of the rare jewels (topaz, big ruby, etc) cap at 10.