Russia and other powers certainly keep a lot of older tanks in reserve condition, but the number of main battle tanks is huge. Here are just the countries that have more than 1,000:
China has 9150
USA has 8848
India has 6464
Egypt has 4624
Syria had 4500
North Korea 4200
Israel 4170
Turkey3778
Pakistan 2924
Ukraine had 2809
South Korea 2381
Taiwan 2000
Belarus 1665
Iran 1658
Greece 1556
Vietnam 1470
Yemen had 1260
Jordan 1250
Morocco 1215
Saudi Arabia 1210
Poland 1009
And on that list, Jordan is the only one in a position to make a sizable contribution to a Saudi effort. Saudi might have dibs on some of the Yemeni tanks, but they frankly have their hands full down there and probably can’t spare any assets from that theater. Paks would have logistic issues, and aren’t in a position to strip the border with India of any assets. They might make a nominal contribution. Turks would love to play ball, but if anything, their logistical problems are bigger than those of the Paks, and they would also be foolish to pull any front-line units at this point unless they were concerned that those units might engage in independent thought.
Likely number of MBTs for such an exercise would probably be 10% of the number that the propagandists are saying.