UFW - allow range of IP addresees?

You need to use a binary number (2,4,8,16, 32) so either you use a bigger or a smaller range. Exactly 20 hosts just is not possible with a bitmask in a single rule:

• 16 hosts (192.168.1.16 to 192.168.1.31):

  sudo ufw allow proto tcp from  192.168.1.16/28 to 192.168.1.48 port 80
  

  Details

  Address:   192.168.1.16          11000000.10101000.00000001.0001 0000
  Rule Mask: 255.255.255.240 = 28  11111111.11111111.11111111.1111 0000
  Wildcard:  0.0.0.15              00000000.00000000.00000000.0000 1111
  
  HostMin:   192.168.1.16          11000000.10101000.00000001.0001 0000
  HostMax:   192.168.1.31          11000000.10101000.00000001.0000 1111
  

• 32 hosts (192.168.1.0 - 192.168.1.31)

  sudo ufw allow proto tcp from 192.168.1.0/27 to 192.168.1.48 port 80
  

  Details

  Address:   192.168.1.0           11000000.10101000.00000001.000 00000
  Rule Mask: 255.255.255.224 = 27  11111111.11111111.11111111.111 00000
  Wildcard:  0.0.0.31              00000000.00000000.00000000.000 11111
  
  HostMin:   192.168.1.0           11000000.10101000.00000001.000 00000
  HostMax:   192.168.1.31          11000000.10101000.00000001.000 11111
  

• 64 hosts (192.168.1.0 - 192.168.1.63)

  sudo ufw allow proto tcp from 192.168.1.0/26 to 192.168.1.48 port 80
  

  Details

  Address:   192.168.1.0           11000000.10101000.00000001.00 000000
  Rule Mask: 255.255.255.192 = 26  11111111.11111111.11111111.11 000000
  Wildcard:  0.0.0.63              00000000.00000000.00000000.00 111111
  
  HostMin:   192.168.1.0           11000000.10101000.00000001.00 000000
  HostMax:   192.168.1.63          11000000.10101000.00000001.00 111111
  

Explanation

I can't give a better explanation than wikipedia

Allow Incoming from Specific IP Address or Subnet To allow incoming connections from a specific IP address or subnet, specify the source. For example, run this command:

sudo ufw allow from 192.168.1.0/24 to any port 22

OR

sudo ufw allow from 192.168.1.0/24 to 192.168.1.48 port 80