How page table base register (PTBR) is used to locate the correct frame using Page Number?
I'll take x86 32 bits PCs as an example. On 32 bits x86 PCs, the CR3 register is used as the PTBR. The CR3 register contains the 4KB aligned address of the page directory. The page directory contains addresses of the different page tables. The x86 processors have virtual addresses which are separated in 3 parts. The 10 most significant bits are the offset in the page directory, the 10 bits in the middle are the offset in the page table and the 12 last bits (to the right) is the offset in the physical page in RAM. Let's look at an example. Let's take address 0x12345678. This is a 32 bits address which translates in binary to 0001 0010 0011 0100 0101 0110 0111 1000. Now let's look at the different parts of this address.
Page directory offset        Page table offset        Physical page offset
   00 0100 1000                11 0100 0101             0110 0111 1000

So in the end this virtual address will translate to page directory entry 0b0001001000 = 0x72 = 114 decimal, page table entry 0b1101000101 = 0x345 = 837 decimal and the offset in the physical page will be 0b011001111000 = 0x678 = 1656 decimal
Depending on what is the content of each selected entries by this virtual address, the actual physical address can be anywhere in RAM. In this example we have entry 114 in the page directory. If at entry 114 is written 0x1234, the MMU of the CPU will look there for the page table. It will use address 0x1234 as the bottom of the page table and then find entry 837 which could contain 0x12345. So the final physical address translated will be 0x12345 + offset_in_physical_page = 0x12345 + 0x678 = 0x129BD. This will be the final physical address once translated using the example that we have here.
On more modern processors you will have 4 levels of page tables so the virtual address will be split in 5 parts. 4 parts of 9 bits to the left and 1 part of 12 bits. Only 48 bits are used for now with the leftover bits to be used later (once there are more page table levels).