If you mean a regular rubber ink stamp to use on normal documents then the Company name, CRO no, Address, Phone number and often VAT umber would be often seen. If you mean the official seal for sealing documents, then the company name and CRO number is usually all that is on that.
My Institute shows the CRO number on their seal ( in the centre with the name round the rim) as affixed to practising certificates. I also see seals with co name only.
Definitely not looking for any disagreement over this point, and I would even agree that putting the company number makes very good sense (as it is the one constant in a company's existence) but I have never come across a seal with a company number on it. Rubber stamps are, of course, a different matter, and can include pretty well whatever the owner wants (within reason!).